From experience, pre-registration is a process where there are games that aren't yet available on stores. If you Pre-register, you could do beta testing for these games. Signing in is usually free, then you get access for that certain Game, before anyone else could. Usually Pre-registration are done by gaming companies. And if you Pre-register too often, or a lot, they even contact you directly and pay you for it.
Geoff's comment is a bit right, you get downloadable content, free items. Although, pre-ordering is when you pay money for it, but Pre-registration is already free. Usually pre-order is for console and PC games, and Pre-Registration are usually on Mobile Gaming. Only a few mmos offer pre-registration
For Nikkicruz's comment, I googled exp game and it is exactly like that. Good guy/girl nikki. I hope this helps your question.