I think I first heard the term memory tax when I was writing about chess. In chess, at the really great levels of play, memory tax is a huge part of what you're dealing with. You don't just have to remember what a bishop does and how castling works - I hate castling. You have to remember whole openings and end-games, and remember how to counter this kind of move, and where that will leave you, and how to regain tempo. Lots of memory tax is chess!
Often these days, though, I find myself thinking about it in all kinds of games. And this point is worth restating up front: when you're reviewing a game you play it in a very different way to how you might play it if you were playing it purely for fun. The biggest difference is focus. If I'm reviewing a game, I plough through it and I don't leave huge gaps between putting it down and picking it up again.
If I'm playing something for fun, though? Oh boy. I might put a game down for a month before returning to it. Six months. A year. And this is where memory tax comes in. It's the eternal question for this particular middle-aged player of games: where was I?