Mishnah 10 in the fourth chapter of Maseches Avos (in some versions it is Mishnah 8) opens with the words "Hu hayah omer" - "He used to say" - referring to Rabbi Yishmael the son of Rabbi Yosi ben Chalafta, who was mentioned in the previous mishnah. In the previous mishnah he taught that a person should do everything in his power to avoid serving as a judge: he should push for a compromise between the litigants, or hand the task of judging over to others if at all possible. Nevertheless, judging is a mitzvah from the Torah, and here the Tanna acknowledges that at times a person will indeed be called upon to judge.
"Al tehi dan yechidi":
If he does come to judge, the mishnah instructs: "Al tehi dan yechidi" - a person should never act as a lone judge, but rather as part of a panel of three, like a regular beis din. Strictly speaking, an expert who is publicly recognized in these areas of halachah is technically permitted to judge alone, at least in monetary law and financial disputes; even so, the mishnah says not to do this.
The reason: "she'ein dan yechidi ela echad" - there is only One who can judge alone, and that is the Holy One, Blessed be He. He alone sees every side of the story and understands the full consequences; He is the judge and He is also the jury, and He knows the witnesses. No other person is like that, and therefore one who comes to serve as a judge should bring two others along with him.
"Ve'al tomar kablu da'ati":
The mishnah continues: "Ve'al tomar kablu da'ati, shehen rasha'in velo atah" - the senior judge, who is recognized and permitted to function on his own, may not say "accept my opinion." Who are "they" and who is "you"? There are two approaches to explaining the mishnah:
The approach of the Bartenura: This is speaking of a judge who is permitted to judge alone, but who chose to bring two other men onto the beis din with him in order to fulfill the instruction of the mishnah. He knows that he is the senior expert and that they are not. What happens if, after the case has been clarified, the two of them hold an opinion that differs from his? To this the mishnah says that he may not tell them: "I am the decisive expert here, and I only included you in order to fulfill the mishnah and out of humility; I know that the law is with me, and you have no right to disagree with me." For there is a rule of "acharei rabim lehatos" - a verse in the book of Shemos which requires that in beis din we follow the majority. Accordingly, when two stand against one, we follow the two and not the one, even if the two are junior and the one is senior. Therefore he may not say to them "accept my opinion": "shehen rasha'in" - they are permitted to disagree, "velo atah" - whereas the senior rabbi is not permitted to force them to agree with him, and if they disagreed with him, the halachah is decided like them.
The approach of Rashi: Rashi understands that we have here a third point. The first point was stated in the previous mishnah - that a person should avoid serving as a judge; the second point is stated in our mishnah - that if he does come to judge, he should not judge alone; and the third point is that when litigants come before him with a dispute that needs to be clarified before a judge or a panel of judges, he may not impose upon them that he is the right man for the job. Even if he believes that he is the greatest expert in the city, and that with him they will receive the fairest treatment and the ruling will be a true ruling - he may not do so. The right belongs to the litigants to decide which beis din they will go to, and they may turn to any beis din they wish, not necessarily his. Therefore "al tomar kablu da'ati" - the expert judge may not tell the litigants to accept his opinion over the opinion of another beis din. "Shehen rasha'in" - they are permitted to bring their dispute to any beis din they choose, "velo atah" - and you have no right to force them to accept you as the judge who decides their dispute.
In summary: In this mishnah Rabbi Yishmael the son of Rabbi Yosi continues his words about the work of judging. First he taught to avoid judging; here he adds that if one does judge - he should not judge alone, since "there is only One who judges alone," the Holy One, Blessed be He. And in conclusion: "Ve'al tomar kablu da'ati" - according to the Bartenura, the senior judge may not impose his opinion on the two judges he brought alongside him, for "acharei rabim lehatos"; and according to Rashi, he may not impose himself on the litigants, since it is their prerogative to choose the beis din before which their case will be clarified.