A blob of joint compound or spackling compound that has been painted over without sanding it smooth to blend in with the wall.
When you patch holes in drywall, you need to sand the compound smooth to blend in with the wall before painting over it. Otherwise, you will leave behind an ugly mess over the patched area, called a Theel blob.
Jordan: Hey, Dad, I would like to paint my room a different color.
Dad: OK, but you're gonna have to do it yourself. I'll buy the paint and other stuff you need, but I'm not doing any of the work.
Jordan: Thanks, Dad! And that's OK, I know how to do it myself. I'm even gonna patch a bunch of holes and dents in the walls.
Dad: OK. Just make sure you sand the compound really well after it dries. I don't want to see any Theel blobs.