Grafting is taking a cutting from one tree, and attaching it to another tree. They will heal together and grow as a single plant.
To accomplish this, we take advantage of the remarkable ability of plants to heal. Imagine if you accidentally cut off a finger, or an arm, and you could just tape it back on, bandage it up, and it would reattach itself.Trees can do that.
When a tree produces fruit and seeds, those seeds won’t grow into a tree identical to the parent tree. Just like you aren’t identical to your parents. So if you plant a seed from an apple, it will grow into a tree, but the apples from that tree will not be identical to the original apple. Often they will be small and inedible “crab apples”. Occasionally you will get lucky, and the seed will grow and produce tasty apples.
By grafting, you can take cuttings from that lucky, tasty apple tree, and clone it, producing an almost infinite number of copies that will produce identical apples.
This is how all apple varieties originated. People planted a bunch of seeds, grew them into trees, and picked the best ones. If you have acres of land and lots of time, that’s an option, but with grafting you know exactly what you’ll get.