That fee is the "standby fee" SWA charges if you want to go standby on an earlier flight than the one you were originally scheduled on; same day travel only.
Unlike the legacy carriers, they don't charge a change fee to reschedule in advance or simply to cancel, and if it turns out that your new flight is actually cheaper than the one you were originally scheduled on, you can sometimes get an actual refund of the price difference (if not, you'll get it as a credit with the airline, to be used against the fare of a future flight.)