Expected value is how much you will make on average from some certain play or decision. Here are a few poker and non-poker examples.

Suppose we flip a fair coin. If it's heads, I'll pay you $5, and if it's tails you pay me $1. Over two flips, you'll average winning $4 (win $5 once and lose $1 once) so the expected value of just one flip would be $2.

Suppose you get all-in preflop for $10 with $0.05/0.10 blinds and you have 60% equity against one player. The total pot is $20.15, and you expect to win 60% of that ($12.09) minus your $10 investment, which makes your expected value $2.09.

For shorthand, +EV means positive expected value (ie: doing a +EV action will yield a profit, while -EV means a negative expected value (the opposite).