Having learned this myself - scion is the portion of a shoot that is last year's new growth. There is usually a " knuckle " or wrinkled collar of sorts that marks the border of last year's growth from the year before. Look for good scion wood on the sunny side of the tree and also at water sprouts ( the straight - up vertical growth that occurs on the top side of horizontal limbs, or near last year's pruning cuts ). The scion is cut just above the " knuckle " so you only have last year's growth. This scion wood can then be grafted onto rootstock to produce a tree just like the tree the scion wood came from.
Guys on this forum trade scion wood by mailing it to each other and basically " cloning " those same trees. Cloning is probably not the exact correct term, but you see the idea. The only way to re-produce a type of tree you want is by grafting ( in some fashion ). Some guys do " t-budding " which I know nothing about. But the point is - if you want a Granny Smith apple - you don't plant a Granny Smith seed. You graft Granny Smith scion onto a rootstock, such as a MM-111 or B-118.
You-Tube has some good videos on cutting scion wood and grafting. You might try typing " Harvesting apple scion wood " into your search bar and clicking search. I don't have any links at hand at the moment, but that should get you to some good ones.