A potful is the amount a pot can hold — a pot full of something. You use it the way you use cupful or handful: it is a rough kitchen measure, not a science number. “A potful of soup” means enough soup to fill the pot, give or take. People say it at home and in stories; it is friendly and a little old-fashioned, not the word on a recipe card that wants grams. Though actually, the size of a potful changes with the pot: a little saucepan and a huge stewpot give you very different potfuls, and everyone knows that, so nobody pretends it is exact.