The seventh perek of Massekhet Yevamot taught about how the relationships that a woman has affect her ability to marry a kohen, and specifically whether she will be able to eat teruma – the tithes that are permitted only to a kohen and to members of his household. The eighth perek, which begins on our daf, continues the discussion of teruma, and teaches about cases where a kohen may not be allowed to eat teruma, and yet his wife will be able to do so because of their marriage.
One example of such a case is an arel – an uncircumcised kohen – who cannot eat teruma himself, even though a woman who marries him will be permitted to eat teruma, by virtue of the fact that she has married a kohen. After all, a kohen who is uncircumcised is still a full kohen, he just needs a brit mila – without which he cannot eat teruma. According to the Gemara, the source for the halakha that an arel cannot eat teruma is the word parallels between teruma and the Passover sacrifice where it is clearly stated that an arel cannot participate (see Shemot 12:48).
Rashi explains that the arel being discussed by the Mishna is someone who does not undergo circumcision because he had brothers who died after they had a brit mila. In such a case we rule that the person should not be circumcised lest he suffer the same fate as his brothers. Rabbeinu Tam argues that such a person is, essentially, free from the obligation of circumcision, and therefore cannot be considered an arel. According to this line of reasoning, an uncircumcised kohen who is not obligated in the mitzva of brit mila, would be allowed to eat teruma. Rabbeinu Tam explains that we must be talking about a case where the kohen is simply afraid of the procedure and refused to have a brit. Only in such a case would he lose the ability to eat teruma.