Each ticket assigns you a boarding group, A through C, say. Each boarding group has 60 people in it. When your boarding group is called (alphabetical order) you line up in order at the gate next to a sign that has your number on it. Passengers 1 - 30 of that group board, then when the last person gets on, 31 - 60 board. Everyone gets to choose their own seat, meaning the later you board the more likely you're going to be in the back in a center (not aisle or window) seat, so boarding slows down tremendously by the last few boarding groups.