Game Mechanics refers to the abilities shown in games (usually video games) that are determined by the rules of the game (examples include hit points, levels, stats, world map crossing in seconds outside cinematics, etcetera) and are not necessarily indicative of a character/entity’s actual abilities.
For example, in many Star Wars games, a capital ship can be destroyed by repeated laser attacks from a single fighter (namely, the fighter reduced the capital ship’s HP to zero over a number of laser attacks that do a certain amount of damage based on coding in the game’s programming).
In an actual in-universe battle, however, this would be impossible as the Force shields would recharge faster than the fighter could damage them (HP doesn’t exist, that’s not how durability functions outside of games. You can’t chip away at something and hope to break on through if the fire power that you are packing isn’t sufficient enough).
Examples include bullets doing minor damage to someone in gameplay in which they are more or less shown to be bullet proof. You once more can’t chip away at durability like this in real life (if you can shrug off a bullet once, you can continue to do so without the threat of a continuous number of them hitting you over an inconsistent interval in arbitrary locations).
Game mechanics are considered non-canon, and using them in an argument is considered fallacious.
In recent years, it seems like numerous individuals have forgotten this basic meaning and arbitrarily call matters that aren’t game mechanics, game mechanics.