Southwest Airlines, like some others, allows a passenger to check-in for their flight up to 24 hours in advance. (Until recently you could only check in after 12:01 a.m. on the day of your flight.) Checking in early usually results in being placed in a more favorable boarding group depending upon how early you do it. You can even print out a valid boarding pass on your home printer. If you're checking luggage, you must still check it with a passenger agent at the airport counter, but if you're flying with only carry-on luggage, you may proceed through security to the boarding gate.
If you do not have the ability to print your boarding pass at home or a resort, it is still a good idea to check-in online to secure the better boarding group, then print your boarding pass at a kiosk at the airport.
Disney resorts will not only print the boarding pass for you, but several airlines will check your bags for the flight at the resort, issue your boarding pass, and the next time you'll see your luggage will be at your home airport, however Southwest does not participate in this program.