In the previous section we learned the halachah that if ten saplings were scattered within a beis se'ah, one may plow the entire field until Rosh Hashanah, and this halachah is not subject to the law of tosefes shevi'is. From here the Mishnah seeks to clarify the definition itself: "Ad eimasai nikra'u netios?" - until what stage in the life of a tree is it considered a sapling and not a tree? The Tanna'im disagree about this question.
The three opinions in defining a sapling:
Rabbi Elazar ben Azaryah - "ad sheyacholu": During the first years of a tree's life it has the status of orlah, and the fruit of the first three years is forbidden to eat. In the fourth year - neta reva'i - the fruit must be brought up to Yerushalayim or redeemed, and through this redemption it becomes permitted to eat. Once the fourth year has ended, the fruit is chullin and completely permitted. Two explanations are given in understanding his words: some explain that he means until the fifth year, when the fruit becomes permitted on its own - without any need for redemption and without bringing it up to Yerushalayim; and others understand that it depends - in the fourth year everything hinges on whether the fruit was in fact redeemed, and if it was not redeemed, one waits until the fifth year, when it becomes permitted automatically.
Rabbi Yehoshua - "ben sheva shanim": As long as the tree is seven years old or less, its status is that of a sapling. The Tosefta explains that his words were said regarding olive trees only; regarding fig trees - until it is six years old, and regarding grapevines - until it is five years old.
Rabbi Akiva - "netiah kishmah": Here too there are two explanations. Some understand that the definition is determined by common speech - as long as people naturally call it a sapling, its status is that of a sapling, and once they begin calling it a tree, it is no longer a sapling. Others understand that he means the first year alone, which is the usual time in which a tree is considered a sapling, and after that it is considered a tree and does not meet the condition of this halachah.
A tree that was cut down and produced new growth:
The Mishnah continues and discusses a tree that was cut down with a stump remaining, and which began to send out new branches - that is, to grow again. The law depends on the height of the remaining stump:
"Mitefach ulematah": If it was cut down to a point where less than a tefach remains, everything that grows from that stump is - "kinetiah", meaning it is considered a sapling in every respect: it is included in the law of saplings that we learned, and the count of orlah also begins anew - the first three years are forbidden to eat, followed by the year of neta reva'i, and the entire process starts from the beginning.
"Mitefach ulema'alah": If it was not cut down that low, and a tefach or more remains - "ke'ilan", meaning it retains its previous status as a tree.
These words are the words of Rabbi Shimon, and the halachah follows Rabbi Shimon.
In summary: This Mishnah clarified the definition of a 'sapling' upon which the halachah we learned depends - Rabbi Elazar ben Azaryah makes it depend on when the fruit becomes permitted to eat, Rabbi Yehoshua sets a measure of years (seven for olives, six for figs and five for grapevines), and Rabbi Akiva sets the definition according to the name people call it. We also learned the law of a tree that was cut down and produced new growth: less than a tefach - it is like a sapling, and the count of orlah begins anew; a tefach or more - it is like a tree, and the halachah follows Rabbi Shimon.