It's how they categorize different run types.
"any%" means you just need to "beat" the game, you can use glitches, and skip large sections and whatnot.
"100%" means you need to collect all things, unlock all of the things, or hit all checkpoints, whatever "doing 100% of the game" means for that game.

There are also "meme" categories for some games like "sex%" in Baldur's Gate 3, where it's the fastest time to have sex after starting a new game. Right now it's just over 2 minutes.