Fair price (also called true odds) is the minimum price you would take on an outcome, based on the model, and still be getting value. The model calculates the probability of the outcome and converts it to that price. If the model gives Over 2.5 Goals a 50% chance, the fair price is 2.00.
Compare that with the market odds. Above the fair price, you are being paid more than the minimum, and the bet is +EV. Below it, you are being paid less, and the bet is -EV. At that same 50% probability, market odds of 2.10 are +EV and 1.90 are -EV.