Mishnayos Avos Perek 1 Mishnah 1

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.

from Sinai: From the One who was revealed at Sinai.

to the elders: who lived long after Yehoshua, and those elders (passed it) to other elders until they reached the beginning of the prophets, who are Eli the priest and Shmuel of Ramah.

to the men of the Great Assembly: There were 120 elders, (including) Zerubavel, Seraiah, Re’elaiah, Mordekhai, Bilshan, who in the days of Ezra were the ones who emigrated from the diaspora in (the period of) the second Temple. Included among them were Haggai, Zechariah and Malachi and Nechemiah ben Chachliah and their associates. They were called the Great Assembly since they restored the crown to its original splendor (Yoma 69b), for Moses said “The great, mighty and awesome God (Deut. 10:17).” Jeremiah and Daniel came but did not say, “Mighty and awesome (Jer. 32:18) (Dan. 9:4),” but they restored it as in the beginning, since they indeed stated both his might and his awesomeness. For were it not so, how could a nation such as this be able to endure in the presence of so many nations.

they stated three things: They stated many things; rather, they said these three things because they contain within them the preservation of the Torah.

"Be discerning in judgment": Where if a case comes before you one, two or three (times), do not say "This case already came before me, and (I ruled) a second and a third time." Rather, "Be discerning," which is to say deliberate before you decide the case.

"stand up many students": (This is meant) to exclude (the words of) Rabban Gamliel, who said "Any student whose exterior is not like his interior shall not enter the Beit Midrash (Berakhot 28a)." We derive from this that we teach Torah to every person; there is no need to inquire after him. [This is the case] so long as it not be known from his way that his actions are corrupt or that he has a bad reputation. Alternately, we may derive that if he raises up disciples in his youth, he should [also] do so in his old age, as is written (Ecclesiastes 11:6), “Sow your seed in the morning, and in the evening do not hold back your hand (Yevamot 62b).”

"and establish a fence for the Torah": A hedge, in order that he not come into contact with a Torah prohibition, such as second degree sexual relationships or rabbinic fences to protect the Shabbat as is written (Lev. 18:30), “You shall keep my guard;” [meaning,] ‘Make a safeguard for my safeguard.’