Casting is the art / trick / science of producing something via a mold. It can be an extremely useful technique for producing multiple copies of a specific piece, like copies of a special button or ornament. It also can be used to "transfer" a piece from one material to another: you can make a model in a material that's easy to work with (like clay or wood) and cast it in a completely different material that's stronger or lighter than your model material (like resin).