A free membership allows a player to play only 20 games simultaneously and when this limit is reached, new games cannot be started until the number drops below 20.
Higher membership levels solve this problem in the following way:
(hide) If you only look at some of the discussion boards on a regular basis you can add them to your favourite boards list by going to the Board page and clicking "Add to my favourite boards". (pauloaguia) (show all tips)