What is rootstock?
Rootstock is the bottom part of a grafted tree which has the roots. The main purpose of rootstock is to control the height of the adult tree and it is broadly categorized as dwarf, semi-dwarf, and full-size/standard. Rootstock can also impart other benefits such as changes to tree vigour (relating to how soon it bears fruit), cold hardiness, and disease resistance.
There are different varieties of rootstock which are grown clonally, and some seedlings that naturally stay so similar genetically they are used for rootstock such as Antonovka and Myrobalan. Similar to grafting to propagate varieties, this lets us ensure reliable results and characteristics from the rootstock we are grafting with. You can see a description of different rootstocks HERE.