Enter the bookmaker's decimal odds and the probability you believe the outcome really has (OddsWasp's predictions are one source of such estimates). The expected value is what you'd win or lose per unit staked, on average, if you could repeat this exact bet many times.
A bet is "value" when your probability beats the break-even probability implied by the odds. Positive EV does not mean this bet wins — it means the price is on your side over the long run.