Google’s trying out a new feature on its app store and the first app to make use of it is the upcoming Terminator Genisys: Revolution game from Glu Mobile. Now, users can pre-register the game from the Play Store. It takes the place of the install/buy button on the app’s Play Store page.
Pre-registering doesn’t mean you agree to buy the game. It’s just a way to tip users that the app or game is already available for purchase or installation. You can even unregister if you change your mind.
Source: Android Police