Stringencies with regard to kodshim - specifically, as they emerge from the biblical text. Also, what can matzah be made from - to fulfill the mitzvah of eating matzah on the first night of Pesach. And also what can you *not* use for matzah (beyond the obvious chametz, which is not under discussion here). Including the different options that kohanim have, which the rest of the people cannot partake of. The gemara articulates the names of the mishnah's list of grains - in the vernacular. Which leads into a discussion of rice and millet, and why they cannot be used for matzah. What's the difference in the way they do or don't become leavened - and a consideration of the phrase, "close to leavening" (does it almost become leavened, or does it become leavened quickly). Plus - other plants that are not grains, and what happens if you grind them and use them as flour, essentially. The debates arise over less obvious plants that are like the 5 grains that do leaven, recognizing that, today, we have far more ways of getting to flour... Note the transition from getting rid of chametz to diving into matzah.