What is a hook? Not a spoke hook like I thought.

The hook is a ridge that faces towards the tire at the edge of a wheel. It’s small but the hook catches the bead of the tire helping with tire retention. If a wheel is hookless then there’s no hook there and instead the top edge of the wheel is flat. Rather than the hook catching the tire bead, the tire presses against the side of a hookless wheel. This is the same way that car tires work.

huh, I thought car rims also had this ridge for the bead to sit in.