Never had retro before?

A retro chess problem is a chess problem that concerns about the past, instead of the future like all those "mate in x" or "play to win/draw" puzzles. And what is more straightforward about questioning the past than the question "what was the last move?".

(1+2) What was the last move?

If you managed to solve it, congratulations! You can do retro problems. It's just complete deductive thinking, armed with the rules of chess. Now you understand what these threads are talking about, even if you can't solve them yet (or *gasp* don't want to solve them).

Note that what happens afterwards (generally) doesn't matter. Here, White has an easy draw, but who cares about that. Some of the threads above have even stranger positions that will never occur in actual play, but hey, this is not a game of chess; this is an art form.