Moses received the Torah at Sinai and transmitted it to Joshua, Joshua to the elders, and the elders to the prophets, and the prophets to the Men of the Great Assembly. They said three things: Be patient in [the administration of] justice, raise many disciples and make a fence for the Torah.

[R' Ovadiah ben Abraham of Bartenura (c. 1445-c.1515)]

Moshe received the Torah from Sinai: I say: Since this tractate is not founded on the exegesis of commandments from among the Torah’s commandments, like the rest of the tractates which are in the Mishna, but is rather wholly morals and principles, and whereas the sages of the (other) nations of the world have also composed books according to the fabrication of their hearts, concerning moral paths, how a person should behave with his fellow; therefore, in this tractate the tanna began "Moshe received Torah from Sinai," to tell you that the principles and morals which are in this tractate were not fabricated by the hearts of the Mishna’s sages; rather, they too were stated at Sinai.

[Rabbeinu Yonah Gerondi (d. 1263)]

Moshe received the Torah from Sinai, etc.: Our rabbis, may their memory be blessed, said (Bava Kamma 30a) "One who wishes to be pious should fulfill the words of Avot (the Ethics of the Fathers), and [some] said about it, the words of Damages (Nezikin)". And since a person ascends the steps of piousness by doing one of these matters, they placed [Avot] in the Order of Damages. And even though they also said to fulfill the words of Blessings (Berakhot) which is in the order of Seeds (Zeraim); because it speaks of the blessings on seeds and fruits, they placed it in that order. Furthermore, since it is the teachings of the Sanhedrin, they placed it in the order of the laws (of Damages); and so [too], all the sages mentioned [here] until Rabban Yochanan ben Zakkai are all from the Sanhedrin.

[Rabbi Israel Lipschitz (1782–1860)]

"From Sinai" - That is to say, Moshe was very humble, such that he received the Torah, which is compared to water that leaves a high place and collects in a low one [Taanit]. And so he received Torah from Sinai, the lowest of the mountains [Megillah 29a]. This teaches that only through humility, the source of all attributes of integrity, can one attain Torah.

[Rabbi Yom-Tov Lipmann Heller (1579 - 1654)]

AND TRANSMITTED IT TO JOSHUA. All that he received, he transmitted to Joshua. But it cannot be said that G-d transmitted the Torah in its entirety to Moses, because not the entire Torah that G-d possessed was transmitted to him, as the fiftieth gate was not transmitted to him, as the Sages expounded (Rosh HaShanah 21b) on the verse (Psalms 8:6) “And you have made him only slightly less than the Lord.” Therefore, in speaking of Moses the mishnah says that he “received” it, i.e. whatever he could receive, he received at Sinai. Thus do the commentaries explain it.

THEY SAID THREE THINGS.

Bartenura writes that of their many sayings, these three were the ones touching on the continuity of Torah. Later in Avot, 2:10, where the mishna also says “they said three things,” Bartenura offers as an alternate explanation that these were the dicta they constantly emphasized, as he writes on Avot 1:2.

(8) And this is the explanation of: Be deliberate in judgment; produce many disciples; and make a fence for the Torah (Avot 1:1).

(9) Be deliberate in judgment— The aforementioned sage must be deliberative, knowing how to speak with each individual. To those who dwell above he reveals the insight of mah, as in “Mah did you see?…” And to those who dwell below he reveals the insight of “the whole earth is filled with His glory.” This is the concept of judgment, of justice, as in “The Lord is judge, He brings down one and lifts up another” (Psalms 75:8). [The sage] must lower the lofty and show them that they know nothing about the Blessed One. But the small, those who “dwell in the dust,” he must lift up; he must rouse and wake them, so that they never despair, because “the whole earth is filled with His glory.” And this is:

(10) produce many disciples— When a person merits such an insight, so that he enlightens both great and small, he is certain to produce many disciples. This is because everyone needs him, on account of his ability to study with all of them, as discussed above. And this is:

(11) and make a fence for the Torah— As in: Silence is a fence for wisdom (Avot 3:13). He must make a fence for his words, at times being silent, as in “Be silent! Thus has it arisen in thought.” By producing many disciples he merits attaining the makifim—i.e., he grasps and understands that which he originally could not grasp. He must therefore make a fence for his words and at times keep silent, so as not to become occupied with makifim that are beyond time, for which there is not enough time to clarify the difficult questions and solutions they contain, as discussed above.