A memory-condition is any element of a game's state that players must know in order to follow the rules correctly, but which is not visibly tracked by any physical component. Here are two examples:
My general rule-of-thumb (which neither Chess nor Hearts respects) is that a memory-condition should not last beyond the scope of a single turn.
If a game is straightforwardly turn-based, you don't need to physically track whose turn it is. If a game consists of more complex phases, you may need a "starting player" marker or some similar device.
If a game contains a hidden but non-mysterious element which players do not need to remember in order to follow the rules correctly, that counts as hidden trackable information rather than a memory-condition.