If you actiavte Waiting List feature, when your event is full, users will be able to join waiting list. In that case, they will be stored and showed in Registrants Management with Waiting List status.
If you want that when someone cancel his registration for the event, users from waiting lists will be moved to registrants automatically (the first users join the waiting list will have highest priority), you can use this plugin.
Just go to Extensions => Plugins, find and enable the plugin Events Booking - Move Registrants to use this feature.