All leagues have a thing called the waiver wire. This is the method used to pick up free agents for your team. The waiver wire is extremely important for picking up hot players to replace your injured or lackluster players. Now the way it works is the following, after every week each team in the league is ranked by how well they did. This is the priority list, and is used to determine who gets what player. What you need to do is pick which players you would like to get and make an order.
Say there are 5 players you want, you are not necessary guaranteed to get any of them, and it is important to put your most desired player on the top of your priority list. You also need to specify which player you would drop to make room for them, but they are only dropped if you get the player you request. At a particular time, usually sometime around Tuesday the waiver wire goes into action and players are picked based on the priority list.
Hoped this help, I know I had difficult time understanding it at first.