We learned in the Mishna (15b) that when Yom Tov falls out on Friday, preparation for Shabbat can be done only if an eiruv tavshilin is prepared before Yom Tov begins. The Ra’avad explains that the idea of the eiruv tavshilin – literally “a combination of foods” – is to prepare a meal for Shabbat at a time when it is permissible, and then food that is made on Yom Tov can be combined with that food in preparation for Shabbat. Beit Shammai are quoted in the Mishna as requiring two types of food for the eiruv tavshilin, while Beit Hillel require just one. All are in agreement that fish with an egg on it is considered adequate.
While this last comment seems obvious, the suggestion is that perhaps this is considered one dish, and it should not be enough for Beit Shammai. The Ri”d explains that this refers to fish eggs – kosher caviar – which at first glance may not appear to be a separate food. The Me’iri explains Beit Shammai’s requirement of two foods as a symbolic meal prepared for Shabbat, for which a single item of food would not suffice.
The Gemara on our daf quotes a baraita that has a different tradition with regard to the opinions of Beit Shammai and Beit Hillel. According to Rabbi Shimon ben Elazar, Beit Shammai and Beit Hillel agree that the eiruv tavshilin requires two foods; their argument is over whether a single dish, like fish prepared with egg, meets the requirement. Beit Shammai insists that two separate foods be prepared, while Beit Hillel rules that such a dish meets the requirement.
Rava concludes the discussion by ruling that we follow Beit Hillel according to the version found in the Mishna. Thus, an eiruv tavshilin really only requires a single prepared food. Nevertheless, the tradition is to use a cooked food together with bread or matza, which would fulfill the requirement according to Rabbi Shimon ben Elazar’s opinion, as well (see the Shulhan Arukh, Orah Hayyim 627:2).