The average profit or loss if you repeated the decision.
Chance of winning × amount won − chance of losing × amount lost. Calling 5 into a pot of 15 and winning 30% of the time: 0.3 × 15 − 0.7 × 5 = +1. In blackjack and video poker, you compare the EV of each action and pick the best.
At the table
Betting 1 on red in single-zero roulette wins with probability 18/37 and loses with 19/37, so the EV is 18/37 × 1 − 19/37 × 1 ≈ −0.027. On average you lose 2.7% of the bet per spin.