The third perek of Massekhet Yoma begins on this daf . From here until the end of the Massekhta, the unique Temple service of Yom Kippur is described, from the first tevilah (ritual immersion) of the kohen gadol, until he completes the avodah (service). This perek specifically is an introduction, as it discusses the preparations and special arrangements made for the avodah, without getting into the details of the avodah itself.
Mishna: The appointed priest said to the other priests: Go out and observe if it is day and the time for slaughter has arrived. If the time has arrived, the observer says: There is light [barkai]. Matya ben Shmuel says that the appointed priest phrased his question differently: Is the entire eastern sky illuminated even to Hebron? And the observer says: Yes.
This was necessary because of an error that had been made once, when the light from the moon fooled the kohanim and they began the avodah before the appropriate time, and the korban tamid (the first sacrifice of the day) had to be destroyed.
There are different opinions about the statement made by Matya ben Shmuel. According to the Rambam, Matya ben Shmuel was one of the tanna’im, and he was disagreeing with the first position in the Mishna, arguing that the question presented in order to clarify that sunrise had occurred was whether it was light in the east all the way to Hevron. Tosafot Yeshanim argues that Matya ben Shmuel was the name of the kohen who was responsible for the lotteries that were done in the Temple (his name is mentioned in that context in Massekhet Shekalim). If we accept this explanation, then he is not arguing, rather the Mishna is describing that after the first sighting of the sun, Matya ben Shmuel followed by asking whether it was light all the way to Hevron.
The Meiri explains that Matya ben Shmuel’s question was whether the kohen watching for the sun could see all the way to Hevron in the south. In any case, the Jerusalem Talmud points out that everyone agrees that the reference was specifically to Hevron because they wanted to invoke the city where the forefathers of the Jewish people are buried.