To fold is to crease or bend something over so it covers part of itself. Before you put your clean sheets and towels away in the closet, you should probably fold them.
Some people drop their clothes on the floor at the end of the day, while others neatly fold theirs. You can call the crease in a folded piece of paper or pillow case a fold too. There are several ways to fold, including to gently mix ingredients, incorporating them without stirring; and to drop out of a poker hand, laying your cards down on the table.