Rosh Hashanah, Chapter 4, Mishnah 1. This mishnah deals with blowing the shofar on Shabbos - when it is forbidden and when it is permitted. The reason we do not blow the shofar on Shabbos, as the Gemara explains, is a rabbinic decree: we are concerned that a person might come to carry the shofar four amos in a public domain on Shabbos, which is forbidden, either in order to learn how to blow it properly or in order to bring it to an expert who knows how to blow.
The text of the mishnah:
"Yom Tov shel Rosh Hashanah shechal lihyos b'Shabbos, ba'Mikdash hayu tokin aval lo ba'medinah" - when Rosh Hashanah falls on Shabbos we do not blow the shofar, as explained, but in the Beis HaMikdash they would blow; outside the Mikdash, however, they would not.
What are "Mikdash" and "medinah":
The general view: "Mikdash" means the Beis HaMikdash itself, and "medinah" includes even Jerusalem.
The view of the Rambam: "Mikdash" refers not only to the Beis HaMikdash but to all of Jerusalem, and "medinah" is the rest of Eretz Yisrael, outside Jerusalem.
The enactment of Rabbi Yochanan ben Zakkai:
The mishnah continues: "Mishecharav Beis HaMikdash, hiskin Rabbi Yochanan ben Zakkai sheyihyu tokin b'chol makom sheyeish bo beis din". Even though there is no longer a Beis HaMikdash, we do blow the shofar on Shabbos in any place where there is a beis din - a Sanhedrin of twenty three. Even if it is not their permanent seat, but they are present there temporarily, that too is considered a place where the shofar is blown.
The dispute between Rabbi Elazar and the Sages:
Rabbi Elazar: "Lo hiskin Rabbi Yochanan ben Zakkai ela b'Yavneh bilvad" - the intent is not specifically Yavneh, but any place where the Great Sanhedrin sits, the high court of seventy one, whose seat was in Yavneh after it left Jerusalem. Any other place to which the Great Sanhedrin moves is also included in the category of "Yavneh". It emerges that Rabbi Elazar disagrees with the first opinion in the mishnah: the enactment does not apply in every beis din of twenty three, but only in a beis din of seventy one.
The Sages: "Amru lo: echad Yavneh v'echad kol makom sheyeish bo beis din" - the law applies in Yavneh, and likewise in every place where there is a beis din of twenty three. According to the Ran this refers to a beis din of twenty three, while according to the Rif even a beis din of three is included.
The practical difference between the first opinion and "amru lo":
The Gemara explains the difference between the last opinion in the mishnah and the first opinion: according to the first opinion, even if the beis din is present in a place only temporarily, the shofar is blown there. According to "amru lo", however, this applies only in a place comparable to Yavneh, where the beis din sits on a more permanent basis.
In summary: in this mishnah we learned that we do not blow the shofar on a Rosh Hashanah that falls on Shabbos, out of concern for carrying in a public domain; that in the Mikdash they would blow even on Shabbos, and there is a dispute about the definitions of "Mikdash" and "medinah"; that after the destruction Rabbi Yochanan ben Zakkai enacted that the shofar be blown in any place where there is a beis din; and that on this point Rabbi Elazar, who limits it to the place of the Great Sanhedrin, and the Sages, who extend it to any beis din, disagree - the practical difference being between a temporary sitting of a beis din and a permanent one.