We are up to Mishnah 20 in chapter 5 of Pirkei Avos (in some versions it is numbered 5:23). The speaker in this Mishnah is Yehudah ben Teima - and in some versions, Rabbi Yehudah ben Teima - who was a contemporary of Rabbi Akiva, so we are moving back somewhat in time.
"Hevei az kanamer, v'kal kanesher, v'ratz katzvi, v'gibor ka'ari, la'asos retzon avicha shebashamayim" - Be bold as a leopard, light as an eagle, swift as a deer and strong as a lion, to carry out the will of your Father in Heaven.
Notice the Tanna's starting point: he does not tell us what a person should do, but rather that a person should possess these qualities and direct them toward serving Hashem. In the broader picture, even traits that are negative under ordinary circumstances - such as boldness and brazenness, as the Tanna himself will say later in the Mishnah - can be channeled toward serving Hashem, and a person is obligated to do so. This is a central idea, and it is no coincidence that the Tur opens his entire codification of halachah with it: a person must direct all of his character traits toward the service of Heaven, in whatever way that is applicable.
Which specific situations does the Tanna have in mind? Here the commentaries take different approaches, and we will go through them one by one.
"Hevei az kanamer":
According to the Bartenura, the namer is a sort of hybrid creature - a species we are not familiar with, and about which we can say nothing. Other commentaries understood that the namer is the leopard, that is, the Arabian leopard: a large cat, about thirty kilograms, which used to be found in Eretz Yisrael. The last Arabian leopard was sighted around the year 2022, and today they have almost entirely disappeared from the land, but in earlier times they were present here. The leopard is known for its boldness: it is not afraid of human beings, it snatches domesticated animals even when people are standing right there, and it attacks animals far larger than itself - fearless and bold.
The message: a person should not be concerned about what others think of him, and this works in two directions. If people laugh at him for being God fearing - let him be bold as a leopard, daring and courageous, not letting it bother him, and doing what needs to be done. So too if he has to stand up and rebuke people for their mistake - it is difficult to stand up in front of others, but if the situation demands it, he must be bold and deal with whatever is required, even if people will not look kindly upon him.
"V'kal kanesher":
The nesher is often translated as an eagle, but it is almost certainly the bird known as the griffon vulture, which is found in Israel to this day. This is the bird that flies higher than any other bird in the world, and indeed Chazal said that the nesher flies above all other birds. It is an enormous bird, with a wingspan of seven to nine feet, and like the other large birds of prey it rides the warm air currents in circles, wheeling through the sky again and again in search of prey, for a long time without tiring.
The idea: a person must be "kal kanesher" in order to review what he has learned. Learning new material is pleasant and enjoyable; review, which is of the utmost importance, is more tedious work, and a person does not always feel like doing it, but would rather skip it. To this the Tanna says: do not feel heavy, with that same heaviness of getting out of bed, but be light and quick on your feet, get up and do your review. And one might add, though I have not seen anyone say this: just as the nesher circles around again and again without tiring, so too a person should review his learning again and again, going around in circles without growing weary, and use this quality for reviewing his studies.
"V'ratz katzvi":
The Bartenura understands that the tzvi is the gazelle, and not the deer with its antlers and relatively heavy body. Gazelles are found in Eretz Yisrael to this day: small, extremely fast and agile, and very timid - they startle easily, and immediately take off at full speed. They were apparently the fastest animals in Eretz Yisrael in that era, and they still are today.
Two explanations are given for this trait:
The Bartenura understands it to mean running toward the performance of mitzvos.
Others understand it to mean fleeing from sin - and this is the explanation I prefer, since I have seen these gazelles in action: at the first sound or the first sight, they run off at full speed. So too a person must be exceedingly swift in fleeing, without falling into the clutches of bad things.
"V'gibor ka'ari" - and mighty as a lion:
Might does not mean courage, but rather as our Sages put it: "Eizehu gibor? Hakovesh es yitzro" - who is mighty? One who conquers his evil inclination - inner strength. Unlike the boldness and brazenness of the leopard, which are directed toward other people, here we are speaking of a person's inner strength, to strengthen himself against his evil inclination. Indeed, the Shulchan Aruch opens at its very beginning: "Yisgaber ka'ari" - a person should strengthen himself like a lion when he rises to serve his Creator in his daily service.
The lion is the apex predator, the strongest animal in the region - most likely the Barbary lion, which no longer exists. But how does the lion in particular embody the trait of might, whose essence is inner strength? I heard an explanation of this that I liked, and I have not found better: all the big cats - tigers, which are even larger than lions, jaguars, leopards and cheetahs - are solitary creatures that live alone. They are at the top of the pyramid, managing their affairs in their own range and territory and enforcing it. Lions are different: they live in a pride, in a group. How can a group of lions, so powerful, live together? That is might, that is self-control: a person should learn to control himself, so that there is enough room for others to function around him, and he does not dominate them the way a tiger or a leopard does. That is "mighty as a lion."
"Az panim l'Gehinnom u'voshes panim l'Gan Eden" - the brazen-faced to Gehinnom and the shame-faced to Gan Eden:
The Mishnah continues: "He used to say" - "Az panim l'Gehinnom, u'voshes panim l'Gan Eden". The brazen-faced, those insolent people, go straight down to Gehinnom; whereas those with shame on their faces - the modest, the bashful and the humble - go to Gan Eden. The contrast is immediately apparent: on the one hand a person must be bold as a leopard in the service of Hashem, yet in general brazenness and insolence, paying no attention to what others think, holding one's head high, doing what one wants and confronting people - that is a bad thing.
Of the entire Jewish people it is said that they are merciful, bashful and doers of kindness. Bashful - people who possess a sense of shame, who are modest and humble, and this is the proper path toward which a person should direct himself. Therefore, if the choice is in his hands, it is fitting that he refrain from using the trait of boldness, even in the service of Hashem. Better that he not let it become part of his personality, as the Mishnah Berurah writes, but rather cling to being a modest, quiet and humble person - in contrast to those brazen-faced people who go to Gehinnom, the insolent ones who may get things done and get in people's faces, but who often end up trampling people, their feelings and their property, and that is a one-way ticket to Gehinnom.
The closing prayer:
The Mishnah concludes with a prayer: "Yehi ratzon milfanecha Hashem Elokeinu shetivneh ircha bimheirah v'yameinu, v'sein chelkeinu b'Torasecha" - may it be Your will, Hashem our God, that You rebuild Your city speedily in our days, and grant us our portion in Your Torah. Yehudah ben Teima, if he is the one who said this line in the Mishnah, lived in the time of Rabbi Akiva, in the generation following the failure of the Bar Kochba revolt. Jerusalem was effectively erased from the map: it was destroyed and its name was changed to Aelia Capitolina, Jews were forbidden to live in its vicinity or even to come near it, and all of Judea was wiped off the map and became Syria Palaestina. There was tremendous pain here over the entire city - not only over the Beis HaMikdash, but over the whole city that had vanished.
There are versions that place this blessing at the end of the chapter, since it is somewhat strange for a blessing to appear in the middle of the Mishnah. If the prayer belongs at the end, then according to the Bartenura it was said by a different person - Ben Hei Hei - and then it is not at all clear that it is connected to our subject. One possible connection: the Gemara in Sotah says that in the end of days, in the footsteps of Mashiach, "chutzpah yasgei" - insolence, brazenness and arrogance will increase and be everywhere. Since the discussion here was about the brazen-faced who go to Gehinnom, their prayer is that they be saved from brazenness, from insolence and from arrogance in the end of days, and that instead we merit, with Hashem's help, to see the rebuilding of Jerusalem the holy city speedily in our days.