According to Biblical law, when a woman experiences her menstrual cycle she is a nidda whether she bleeds only once or many times over a period of seven days. At the end of seven days she immerses in a mikva and is rendered ritually pure. A zavah is a woman who experiences a flow of menstrual-type blood during a time of the month when she is not due to experience menstrual bleeding. The eleven days that follow are called yemei zivah; if she experiences vaginal bleeding during that time she is rendered a zavah.
The laws of zavah differ from those of a nidda. If a zavah experiences bleeding just once or twice during that period, she is deemed a zavah ketanah who will “keep watch a day for a day” – she must check that she is free of bleeding one day for each day that she bled. After experiencing bleeding on a third day, however, the woman is considered a zavah gedolah and is obligated to wait a full seven days without bleeding. At that time she can immerse in a mikva and she will be permitted to her husband. The next day she must bring a sacrifice as part of her purification process, which will allow her to enter the Temple and consume sacrifices (see Vayikra 15:25-29).
The Mishna on today’s daf discusses cases where a woman experiences a discharge of blood at the end of the eleven day period of yemei zivah. In such a case, she cannot become a zavah gedolah who needs to wait a week, since subsequent bleeding cannot take place during the eleven day period that has now ended. If she immerses in the mikva that night without waiting at all, and engages in sexual relations with her husband, Beit Shammai rule that since she did not wait a day as a zavah is required to do they have transgressed; they both are ritually unclean and they must bring a sin-offering. Beit Hillel disagree, ruling that she need not wait the additional day, since in any case it is after the eleven day period of yemei zivah.
If the woman waited until the following day to immerse, Beit Shammai agree that there is no biblical prohibition; Beit Hillel refer to the husband as a gargeran – a sexual glutton.
Rashi explains that although according to Beit Hillel there is no prohibition against engaging in sexual relations under these circumstances, the reference to sexual gluttony stems from the fact that this kind of behavior can lead to an actual prohibition. If the couple gets used to engaging in relations following immersion on the day following bleeding during yemei zivah without waiting a full “clean day,” at some point they may do so even during the eleven day period of yemei zivah, which the Torah forbids.