To preheat is to heat an oven (or sometimes a pan or grill) to the right temperature before you put the food in. Recipes say “Preheat the oven to 180°C” because many cakes and breads need that heat already waiting; if you put them in a cold oven, they cook in a different way. It is an ordinary kitchen verb, used in cookbooks everywhere. You can also meet a noun, a preheat, for that first heating stage, but the verb is what students need. Though actually, you preheat the oven, not the cake: the food is what you cook after the heat is ready. People stretch it a little (“preheat the pan with a bit of oil”), which is the same idea — get the heat ready first. It is not a fancy or scientific word; it is just the cook’s “do this before.”