The Mishnah, compiled and edited under Judah ha-Nasi around 200 AD, organizes Jewish oral law into six orders covering agriculture, festivals, family law, civil and criminal law, temple ritual and ritual purity, and within that overwhelmingly legal structure Pirkei Avot stands out as the one tractate containing no legal rulings at all. Instead it records a chain of ethical maxims transmitted, in the text's own presentation, across an unbroken line of teachers reaching back through the rabbis of the Mishnaic era, the earlier sages of the Great Assembly, and ultimately to Moses receiving the Torah at Sinai, a chain of transmission the tractate lays out explicitly in its opening lines as a way of grounding rabbinic authority in direct, unbroken succession from revelation itself.
The sayings themselves range from terse aphorisms, Hillel's famous formulation that what is hateful to you, do not do to your fellow, that is the whole Torah, the rest is commentary, go and learn it, to longer reflections on humility, study, right conduct in business, and the proper use of one's time, giving later readers a compact anthology of rabbinic ethical thought that requires no specialized legal training to understand or apply, unlike most of the rest of the Mishnah.