The first Mishnah on our daf discusses the case of a house with a plain wooden roof that has no tar or other kind of covering. All of the tanna’im agree that some type of preparation must be made in order for the house to be used as a sukka. They disagree on what needs to be done:
In the case of a roof made of boards that are four handbreadths wide upon which there is no coat of plaster, Rabbi Yehuda says that Beit Shammai and Beit Hillel disagree with regard to the manner in which to render it fit. Beit Shammai say: One moves each board, and then it is considered as though he placed the board there for the sake for the mitzva of sukka, and one then removes one board from among the boards and replaces it with fit roofing. Beit Hillel say: One need not perform both actions; rather, one must either move the boards or remove one from among them. Rabbi Meir says: One only removes one from among them and does not move the others.
The concept of mefakpek – moving the boards on the roof – is to remove the Rabbinic prohibition called gezeirat tikra, a concern that someone will sit under a real roof. By moving the boards around, the person indicates his awareness of the fact that he cannot fulfill the commandment of sukka by sitting in a house under a real roof. In so doing, he succeeds in removing the gezeirat tikra. (See further discussion of this issue in the Ramban, Ri”f, Ritva and Me’iri.)
The word mefakpek meansto move something from its place by shaking it. In modern Hebrew the term has been “borrowed” to refer to conceptual issues, as well, where it means to question an accepted idea. According to the Rambam, what is accomplished by shaking the boards is the removal of the nails that are holding them in place in the roof. Other rishonim (like Rabbi Natan Av HaYeshiva and the Peirush Kadmon on Sukka) say that shaking the boards shifts them around, leaving room for additional s’khakh to be added.