2 point is having your butt totally out of the saddle, like jumping position (although there are different jumping positions). It's called 2 point because there are only 2 points of contact with the saddle - your stirrups.
Then there is 3 point. Totally sitting, think dressage seat. 3 points of contact - your stirrups and your butt.
Half seat is in between. You're not sitting with full contact and you're not out of the saddle with no contact. You make contact with the seat of the saddle every stride as your horse canters along.
A good half seat visual for me is while sitting fully in the saddle, picture someone tugging on your heels. This raises your hip bones and seat just enough to where it's still touching the saddle, but in a much lighter capacity. The idea is to free up your horse's back for a nice forward gait while still being secure in the tack.