Ellen: Did you see the new building down the road? It’s finally finished.
Opal: Yeah! But what an eyesore!
Ellen: I know! It’s a real monstrosity.

An eyesore is something really ugly. If your neighbor puts up a big ugly decoration in his yard, you might complain that it is an eyesore. The word sounds like it makes your eyes hurt (sore) to look at it, but ugly things cannot actually cause eye pain.