Welcome to Maseches Yoma, chapter 1, mishnah 1. The opening mishnah of the maseches deals with the preparation of the Kohen Gadol for the Yom Kippur service: his separation from his home seven days before the day, and the preparation of a replacement in case something goes wrong.
"Shivas yamim kodem Yom HaKippurim mafrishin Kohen Gadol mibeiso l'lishkas Palhedrin":
Seven days before Yom Kippur the Kohen Gadol is separated from his home - the place where he normally lives together with his wife - and he parts from her and comes to the Palhedrin chamber, which is an area or room in the Beis HaMikdash. There he spends seven days preparing for the Yom Kippur service.
The reason for the separation:
The Yom Kippur service can be performed only by the Kohen Gadol and by no one else, and therefore we must be certain that he is tahor. If he were to remain at home with his wife, there is a concern that she would become a niddah, that he would be with her, and that he would become tamei in this way - and we do not want that to happen. Therefore he is separated seven days in advance, and he spends the time in the area called the Palhedrin chamber.
The source for this halachah, that the Kohen Gadol must be separated, is learned from the seven days of the milu'im - the days of the dedication of the Mishkan, during which Aharon and his sons were required to remain in the Mishkan for seven days and seven nights.
"Maskinin lo Kohen acher tachtav":
The mishnah continues: another kohen is prepared to take his place, "shema ye'era bo pesul" - lest he become tamei in some way and be unable to perform the service. That replacement kohen, however, is not separated from his home, because the separation from home is not indispensable. It is not an absolute obligation but a preferable practice done l'chatchilah, as in the first case; and if the separation was not carried out, the Kohen Gadol is still fit for the service - and therefore the replacement too is fit even though he was not separated.
The opinion of Rabbi Yehudah:
Rabbi Yehudah says: "af ishah acheres maskinin lo" - another wife is also prepared for him, "shema tamus ishto". And why is this a concern, and why must he have a wife? "Shene'emar: v'chiper ba'ado uv'ad beiso" - the verse says, within the description of the service, that his offering atones for him and for his household, and as the mishnah explains: "beiso zo ishto" - his household means his wife. From here we see that he must have a wife, and if his wife were to die he might become disqualified.
The response of the Chachamim:
"Amru lo: im kein ein ladavar sof" - if we are going to worry that a person might die, there is no end to such concerns: perhaps the second wife will die as well. The principle is that death, though unavoidable, is not a common occurrence at any given moment, and therefore we do not worry that the kohen will be disqualified because of it. A replacement Kohen Gadol, on the other hand, is indeed prepared, because tumah is far more common and can easily happen - and for that we do in fact take precautions.
In summary: in this mishnah we learned that the Kohen Gadol is separated from his home seven days before Yom Kippur and brought to the Palhedrin chamber, in order to preserve his purity for a service that no one else can perform, and we learned this halachah from the seven days of the milu'im. We also learned that another kohen is prepared to take his place out of concern for disqualification - but he is not separated from his home, since the separation is not indispensable. Rabbi Yehudah adds that another wife is prepared for him as well, based on the verse "v'chiper ba'ado uv'ad beiso" - "beiso zo ishto"; and the Chachamim rejected his words: "im kein ein ladavar sof," for death is not common and we do not take it into account, whereas tumah is common, and because of that concern a replacement kohen is prepared.