Southwest lets passengers who need extra time (elderly, handicapped, people with small children, etc.) board before the herds; ergo, preboarding. Pretty much any airline will let you do the same thing, but it's an extra-big deal with Southwest because they won't assign seats so once at the airport it's a free-for-all.
That's still boarding. Preboarding is what you do before you board, in my case having a Bloody Mary.