Theoretical return is the payback percentage worked out from a slot's maths model: every possible outcome, multiplied by its probability, summed. It is the figure published as the game's RTP.
"Theoretical" is the important word. It describes the game's design over an effectively endless run of spins, not the result of a session, a day or a single player's lifetime of play. Actual returns measured over a limited number of spins will differ from it, and the gap is driven by variance.
Test houses verify the theoretical return by checking the maths and by simulating very large numbers of rounds, and operators monitor live performance against it. A game paying far below its theoretical return over a short spell is not faulty; a large, persistent gap over a huge sample is what regulators and test houses look for.
Some games state the theoretical return including a jackpot contribution, so read the help screen for the breakdown.