Mishnah 4 in the first chapter of Maseches Eduyos, along with the three mishnayos that follow it, are especially fascinating: these are mishnayos that deal with the Mishnah itself - with how it operates and what considerations guide it. Here the Mishnah reflects on its own method, and that is a fascinating matter.
In the previous mishnah we learned that Hillel transmitted a certain tradition, and according to many commentaries he ultimately abandoned his position and followed the tradition of the two weavers. In the two mishnayos before that as well, we saw opinions of Hillel and Shammai that were rejected in favor of the opinion of the Chachamim. The question therefore arises: what is the purpose of mentioning the opinions of Hillel and Shammai in the first place? Is it merely to embarrass them? Their opinion was rejected, so why raise it again?
The words of the Mishnah:
"U'lamah mazkirin es divrei Shammai v'Hillel l'vatalah?" - Why do we mention the names of Hillel and Shammai for nothing, with no benefit at all?
"L'lamed l'doros haba'im" - The mention is meant to teach future generations that a person should not stand firmly on his own words: he should not be locked into his opinions and dogmatic, insisting on what he thought without being willing to move from his position.
"She'harei avos ha'olam lo amdu al divreihen" - Even the greatest figures of our tradition, the fathers of the world in the literal sense, the pillars of the universe, were not locked into their approach and were willing to move from it.
Two ways of understanding this:
Hillel and Shammai were the ones who retracted: This is the straightforward reading, based on the previous mishnah. Hillel and Shammai held traditions, and nevertheless they abandoned them in the face of the tradition of two simple men - the weavers from the Dung Gate, who testified in beis din and whose testimony established a halachic fact. Hillel and Shammai retracted, and this teaches you that a person must be ready and willing to retract.
The Chachamim were the ones who retracted: Some understand that it was not Hillel and Shammai who retracted, but rather the Chachamim - the rest of the beis din. Even though they knew that the great Hillel and Shammai had said what they said, since we have before us two valid witnesses testifying about a tradition, we accept their tradition and it overrides the tradition of Hillel. Even if Hillel said in the name of his teacher that "a sheminis must be left overnight," since the two weavers testified that this was not the tradition, we accepted their testimony. It turns out that the Chachamim who ruled on the case followed the simple weavers and not the exalted Hillel and Shammai, to teach you that a person must be flexible and not dogmatic.
The staves of the Aron:
Let me add a point that Rabbi Yitzchak Berkovits would always note: one of the negative commandments is the prohibition against removing the badim, the poles designed for carrying the Aron in the Mishkan. On the face of it this is puzzling: if the entire Mishkan revolves around the Aron that holds the Luchos, why is it forbidden to remove the carrying poles? After all, no one is about to carry it, and when something is fixed in its place, it is fixed. The answer is that the Torah and everything connected to it must always be ready to move for the sake of truth. One must never be locked into a position.
In summary: This is indeed the straightforward understanding of our mishnah - a warning to a person not to be so convinced of his own correctness, but rather to be open to the truth and to hearing new ideas, and willing to change his mind and his position when the evidence and the truth demand it.