Positive Expected Value. It comes from casino bonuses really when you are working out what you should make on average, but of course cant ensure you will.
If you are tossing a coin and have a 50/50 shot of winning, but when you win you are paid out 2 to 1, then that offer is +EV. You "should" make a profit if you did it enough times, but of course you may lose it was a 50/50 chance. But when you win its a disproportionate amount.
I suppose a bookmaker example might be getting your satke back if your horse loses to the favourite (PaddyPower do that alot). There is no garuntee your horse will lose to the Fav but if you have backed and layed to prevent any loss regardless it might happen and so that is +EV, if you did it enough times you will profit.
If any offer is +EV then do it, if is -EV then dont. Thats a good rule to work to.