In games, we have different kinds of control over the story and/or assets and the related mechanics. The term operator simply means the player and what the player is able to control in the game.
The diegetic actions of the operator is the literal manipulation of the game within the confines of the actual elements. If you tell the character to move up, that’s a diegetic action. Using Rimworld as an example, it would be clicking on a character and making them perform an action.
Non-diegetic actions of the operator include working outside the confines of the game itself, though you’re still using the game. A non-diegetic action in this case would be pausing the game, speeding it up, setting the storyteller AI to a harder or easier setting, and so on.
Video Games Deconstructed 