The event in which, while gaming, something unexpected happens and you need to explain why.
Alternatively it is used to explain why you are doing something difficult or boring but provides rewards on completion.
Finally it can be used as a good old exclamation in everyday life.
*Character is floating in mid-air*
Observer: What!? How is that possible!?
Observer: Why are you going all the way over there for a sword?!
Gamer: because SCIENCE!