Ah, it's mostly where you take one character, we'll call him Arnold, and make his life a living hell for no reason.
For example, Arnold (who is a minor here) might get a call saying that his parents died, then moves in with a nice foster family. They can't keep him because of plot and he moves to an abusive family. Then we find out that his nice family was killed because they took him in. (I'm not shitting you, this has happened in a few stories.)
Then, while he's in school, he gets bullied severely, to the point of suicide. He gets talked down from it by a random character telling him that life gets better, even though it's a WHUMP fic so it'll only get worse.
I could add more bad things in Arnold's life but I think this describes WHUMP pretty well.
Here's a defenition:
Whump -- refers to stories in which physical or emotional pain is heaped on a favourite character, often repeatedly and brutally. Very similar to Hurt/Comfort, but differentiated mainly by motive: H/C fics exist to provide one character the chance to offer comfort to the injured one, whereas whump fics are usually written more for the sheer pleasure of seeing the whumpee battered and bruised. (I don't agree with this next sentence) Different from Angst fics because whumped characters don't really suffer for all they are hurt by a gleeful author.
Edit: You see this a lot in MCU Spider-Man stories on AO3.