Southwest Airlines does not have assigned seating. Instead, they use a unique boarding process called "boarding by group," which allows passengers to board the plane based on their boarding group and number, which are assigned at check-in. Passengers are then free to choose any available seat on the aircraft.
To determine your boarding group and number, Southwest Seat Selection assigns each passenger a boarding position when they check-in for their flight. You can check-in online starting 24 hours before your flight, or you can check-in at the airport on the day of your flight. The earlier you check-in, the earlier your boarding group and number will be, and the better chance you'll have of getting the seat you want.
It's worth noting that Southwest also offers an option to purchase Early Bird Check-In for an additional fee, which can help you secure a better boarding position and increase your chances of getting the seat you want.