Grafting joins a scion (the desired variety) to a rootstock (chosen for vigor, disease resistance, or size control). The two grow together as one plant.
Most commercial apple trees are grafted — seed-grown apples don't breed true (an apple from a Honeycrisp seed will not produce Honeycrisp fruit). Grafting is the only way to propagate a specific variety reliably.
Rootstock choice determines tree size, precocity, and disease susceptibility more than scion choice does. The same variety on dwarfing rootstock produces fruit in 2–3 years; on vigorous rootstock, 6–10 years. There is no free lunch: dwarfing rootstocks require more support and irrigation.