A payout percentage or theoretical return to player is the percentage of your theoretical payback in that game. This is not a representation of how much or how often you should expect to win during a gameplay session - this is an average payback over a significant amount of gameplay.
It is a theoretical calculation based on statistical probabilities. The RTP of online slots can vary from game to game, but it is typically displayed as a percentage. For example, if a slot game has an RTP of 95%, it means that, on average, for every $100 wagered, the game will pay back $95 to players over time, randomly.