It's a bit loosely defined, but it's a term for a new "genre" of indie game, turned around fairly quickly, with an emphasis on multiplayer, usually proximity voice chat, sold for cheap. They're games that are meant to let you and your friends create really fun moments together.

It's "slop" because these aren't usually very deep games, with fairly simplistic art, usually sold for less than 10$, but honestly they're fantastic, and I'd highly recommend many of them.

My favourites would probably be Lethal Company and Peak, but there's also Content Warning, RV Life, REPO, Gamble with your Friends, and tons of others.