EV = Expected Value
http://en.wikipedia.org/wiki/Expected_value
Simplified if you take a slot return of 94% that would be your long term EV. Of course this is negative, in the short term this can vary wildly in the long term your EV should average out to what is expected of the return of the game. Video Poker is probably the best game to guarentee over the long run you get your EV a slot only has to pay one player a big win to hit its % return whereas the nature of video poker played long enough every player will get their 90+% back. Full Pay deuces for eg has a return of 100.7% making it a positive game. In the long run you could earn a living playing this game should you play perfect mathematical stratergy