Starch is the stuff that makes your shirt collar look crisp and fresh. It's also the complex carbohydrate that potatoes and rice — foods that are called starches — have lots of.
Starch can also be a verb. You can starch your shirt by adding starch, which is a white powder, to the rinse water. You can also get the kind of starch that comes as a spray, which you squirt on your shirt while you're ironing it. Cornstarch, which is also a white powder, is used to thicken sauces or make egg whites stand in stiff peaks when you beat them.