Each time you download/install a purchased game, it needs to be "activated" in order to be able to be played without interruptions.
When you initially purchase a game, it has a limit of three activations. This is in place in an attempt to combat game piracy, and in actuality, it's more of a temporary limit. If you receive an "Activation Limit" message, simply create a ticket, and we will be more than happy to reset the activation limit for your game.