Southwest's boarding policy is often called "cattle call" style, because there are no assigned seats. When you check in, which you can do either online the morning or your flight after 12:01am or at the airport when you check your bags, you are assigned to one of three boarding groups: A, B, and C. The groups are assigne in a first-come, first-served fashion, so the earlier you check in the better chance you have of getting in Boarding Group A.
They board the plane in this order: First 'preboarders' are allowed to get on. Preboarders are anyone with disabilities or anyone with small children in car seats, but there have been complaints of abuse of this system (one kid but 12 adults all get on the plane as preboarders, or people who don't appear to have a disability, though this is very hard to judge just by looking at someone).
After the preboarders, groups A, B, and C get on in order.
Since there are no assigned seats, the earlier you can get on the plane the better selection of seats you will have. Naturally, the front of the plane fills up fastest, though I have found that many people on any airline tend to block the aisle forever while trying to fit oversize luggage in the overheads, making it difficult for later boarding groups to get farther back into the cabin.
Whether you check in online or wait till you get to the airport, you should arrive at the airport 2 hours before your flight. It doesn't always take 2 hours to get checked in and get to your gate, but it's always good to have that extra time and not be hurried. Since I travel solo, I can usually check my luggage, get through the security lines, and get to my gate in about 30 minutes, unless there's a long line at the security gates.
