Outer Wilds is a hard one to describe. I don't think that describing the premise itself is TERRIBLY hard, but you really just have to play it to see why people think it's so awesome. I didn't find it as life changing as some people do, but it was still a damn good game. "So it's a puzzle game... but not really, and you just go to planets and explore and read things to decipher what is causing a certain anomaly."
Neon White is a kind of weird one too: "It's a FPS game where guns are cards and you can discard said cards to gain special abilities and you have to figure out how to kill all the enemies and get to the end". It's just crazy how it's like a parkour FPS game, but still has SHOOTING unlike something akin to Mirror's Edge.
I think the craziest one of all is Dwarf Fortress though. I don't even know where to begin describing that game. I should pick it up again, it's just complex as hell. The way I would pitch it to someone is "There was a bug where cats were dying in taverns, and it turns out it was because they were stepping in alcohol, and then when they would clean themselves by licking their paws, they'd die of alcohol poisoning. Yeah so it's a simulation/management game with THAT level of complexity."