I think it's mostly pretty self-explanatory? It's when you want to talk about how the gameplay works on the order of what you're doing, what output the game gives you and what your input is, what choices you're making, etc, in the space of one second, ten seconds, maybe thirty seconds at most, as opposed to what you're doing when you scale out to "gameplay over the course of a quest" or "gameplay over the course of a stage" or "gameplay over the course of an hour" or whatever.
If I go get in a fight with an enemy in an RPG, the 'moment to moment' gameplay refers to what happens specifically within that fight (or even within a portion of that fight). It ceases to matter whether it's a fight that takes place in the middle of some sprawling non-linear open-world structure, or whether it's a series of linear-but-connected areas like in Dark Souls/Castlevania, or whether it's a detached standalone stage like Mass Effect 2/3, or even if it's just a series of fights strung together without any substance in between, like Final Fantasy Record Keeper. The specific feel and flow of the combat and its mechanics can be compared directly, because you're no longer couching it in a holistic "Well, that's just one small part of the whole package" approach.
