The crust is a hard layer that forms on the outside of something, like a loaf of bread. Little kids often like their sandwiches with the crust cut off.
You’ve probably eaten lots of crusts — bread crust, pie crust — any food that’s hard on the outside and soft on the inside has a crust. Crust is also a verb meaning “to form a hard outer layer.” Crusting happens with more than just food, like when a layer of dirt crusts your clothes after a romp in the mud. The Earth has a hard crust too. Little kids don’t like to eat that either, usually.