Mishnah 6 deals with the following case: Shimon is in possession of a field and holds a bill of sale testifying that he bought the field from Levi. Reuven comes and contests the ownership - the Mishnah calls such a challenger an 'orerin' - claiming: "Levi, who sold you the field, stole it from me, and in truth it is mine."
The difficulty is that Reuven himself signed as a witness on that very bill of sale by which the field passed from Levi to Shimon. That signature casts doubt on his claim: if the field really was stolen from him, why did he sign as a witness to its sale? On the face of it, the two simply do not fit together.
The dispute between Admon and the Sages:
Admon says: "Yachol hu sheyomar" - Reuven is entitled to argue: "HaSheni noach li v'harishon kasheh heimenu" - "The second is easy for me, and the first is harder than he." In other words: Levi, who stole from me, is a harsh and powerful man, and I have no ability to contend with him; Shimon, by contrast, is a man with whom it is easy to litigate. I therefore signed as a witness to the sale, so that the field would leave Levi's hands and pass into easier hands, and from there I would sue Shimon in court and recover what is mine.
And the Sages say: This is no claim. By signing as a witness, Reuven gave up his claim, since in the very act of signing he effectively admitted that the field is not his.
Making it a landmark for another:
A further case: Reuven did not sign as a witness to the sale between Levi and Shimon, but rather owned a field adjacent to the field in question, and sold it to someone else. In the deed of that sale he listed the field in question as a boundary and a landmark, and attributed it to Shimon (or to Levi) - meaning that he treated it as a field belonging to another and not to himself.
In this case everyone agrees, Admon included, that he has forfeited his right and lost his claim. For he had no reason or need to identify the field as Shimon's field and to use it as a boundary, if it truly is his own field.
In summary: In this Mishnah we learned two cases in which the challenger's actions seemingly contradict his claim: one who signs as a witness on the bill of sale, about which Admon and the Sages disagree - Admon upholds his claim because "the second is easy for me, and the first is harder than he," while the Sages regard his signature as a waiver of the claim; and one who makes the field a landmark and boundary in another sale, about which all agree that he has forfeited his right.