A rootstock is the root part of your tree. It can be as simple as root grown from an apple seed or a cloned root started from cuttings. However rootstocks grown from seed will always create a full-sized tree. Cloned rootstocks were selected for their superior characteristics such as size and adaptability to certain soil types and are predictable in how they perform. For that same reason named varieties with superior traits such as a Honey crisp apple are grafted or joined on to a rootstock.