"...I don't get it..."
A horse that is on the bit is basically getting a horse at a certain body position. If the horse is truly on the bit, they won't be pulling or rooting down with their head, flex their neck through the poll, their withers, their backside, and they will round up and ball up, and come up underneath them.
If the horse is above the bit then their head is raised higher than the rider’s hands. This is normally caused by the rider holding the reins too tightly which applies too much pressure to the bit. Not only is it uncomfortable for the horse but it’ll reduce your control.
Behind the bit is talking about the horse with their head curled too far down, lacking acceptance of the bit.
