Southwest has one of the easiest boarding systems to understand. You line up based on the number on your boarding pass. There are three groups, A, B and C, with passengers numbered 1-60 in each letter group. There’s no assigned seating. When you board, you can pick any available seat. Thus the advantage of getting the lowest boarding group and the lowest number in that group possible.
If you get lucky enough to have a low number, you may be able to snag this seat on a Southwest plane.
So while there are no assigned seats, Southwest has come up with a number of ways to sell preferred access to the lower boarding numbers.