Flour is a fine, powdery ingredient that's used to bake bread and cake and is made by grinding wheat or other grains. A traditional pound cake is made with a pound of flour.
You'll need flour when you bake almost anything, and while flour is usually made from wheat, you can also buy rice flour, corn flour, and may other kinds. When you sprinkle flour on the counter before kneading bread dough, you flour it. Flour comes from its homophone, flower, from the sense of "the finest or loveliest part of the meal," just as a flower is the finest part of a plant.