We are up to the second-to-last mishnah in chapter 5 of Pirkei Avos - mishnah 22 (in some editions it is numbered as mishnah 25). Its author is Ben Bag Bag. That is not his real name but a nickname; the same is true of Ben Hei Hei, the author of the mishnah that follows. Both were converts, and these names concealed their identity, since there was genuine danger involved - those who wished them harm were searching for them. Ben Bag Bag lived in the time of Hillel the Elder.
It is worth noting that these two mishnayos - Ben Bag Bag's and Ben Hei Hei's - are written in Aramaic, which is highly unusual in this tractate. Perhaps that was the language they spoke, and perhaps they wanted their message to be understood by the common people, who spoke Aramaic rather than Hebrew, so that it would be accessible to everyone.
The text of the mishnah:
"Hafoch bah va'hafoch bah, dechola bah, u'vah techezei, v'sav u'vlei bah, u'minah la tazua, she'ein lecha middah tovah heimenah" - Turn it over and turn it over, for everything is in it; look into it, grow old and worn out in it, and do not move away from it, for you have no better portion than it. All of this refers to the Torah.
Ben Bag Bag is speaking here about the greatness of Torah and Torah study, about the obligation to learn Torah throughout all the days of one's life until the very end, and about focusing one's efforts - especially one's intellectual efforts - on Torah study, to the exclusion of other pursuits.
Explanation of the phrases:
"Hafoch bah va'hafoch bah" - In the time of the Mishnah, Torah was learned from scrolls, and turning the scroll meant rolling it forward and backward. But the inner meaning is to delve into it and reach its depths. What a person learns at age five, ten or fifteen is nothing like what he is capable of learning at forty-five, fifty or fifty-five - and therefore he must keep searching in it again and again.
"Dechola bah" - Everything is in it: everything worth knowing, and everything there is to know, is found in the Torah. Many commentators even speak of how a great deal of what we usually call secular studies can be learned from within the Torah. The Maharal explains this based on the statement of Chazal, "Istakel b'oraisa u'vara alma" - the Holy One, blessed is He, looked into the Torah and created the world. The Torah is, as it were, the DNA of the universe and its source code, and through understanding the Torah itself one can understand the world.
"U'vah techezei" - And in it you shall see. The Rema explains that a person will see the truth and understand what is right through his understanding of the Torah. Another approach explains that a person sees by means of it - he looks at the world through the framework of the Torah, a kind of "Torah glasses." That is the way a person is meant to see the world.
"V'sav u'vlei bah" - Grow old and worn out in it. The Torah will still be here after a person is gone, and there is never a shortage of what can be learned from it; therefore he must keep learning until the very end.
"U'minah la tazua":
This is a warning against the person who says, "I have already learned all the Torah I need to learn, and from now on I can move from our body of wisdom to other sciences and other studies." The position of the mishnah is that one does not move away from it.
The Gemara relates that Rabbi Yehoshua was asked when it is permitted to teach Greek, and he answered that it is possible at an hour that is neither day nor night - for it says, "You shall meditate on it day and night," and a person must learn Torah day and night, so he will have to find some other time for it. Others permit it but place limits on it, similar to the fact that one may not learn Torah in certain places such as a bathroom and the like. Indeed, Jewish life contains various traditions regarding the role of secular studies and how much of them is appropriate, and many of our sages were genuine experts in fields outside of Torah. But whatever the reality may be, the position of the mishnah here is that one does not move away from it, from the Torah itself.
"She'ein lecha middah tovah heimenah" - You have no better portion than it. In terms of making the most of one's time, and in terms of what a person ought to be learning, there is nothing better than Torah, not in this world and not in the World to Come. For this reason a person must focus his efforts specifically on Torah study.
In summary: In this mishnah Ben Bag Bag teaches the greatness of Torah and the obligation to learn throughout all the days of one's life: to turn it over and delve into it at every age, to know that everything is contained in it since the world was created through it, to see the truth in it and through it, to grow old and worn out in it, and not to move away from it to other pursuits - for there is no measure and no portion better than it, in this world and in the World to Come.