Mom: There is so much of the dog's dirty ball marks.
Me: *umm ohh no* HAHAHAHAHAHAHAHAHA dirty balls!
Mom: There is so much of the dog's dirty ball marks.
Me: *umm ohh no* HAHAHAHAHAHAHAHAHA dirty balls!
Okay, so basically, the mom is complaining about the dog leaving dirty marks from its ball. But the joke teller is like, "Oh no!," pretending that Mom is talking about something else entirely: dirty balls, ya know, like testicles. It's funny because it's unexpected and a bit naughty. You probably wouldn't get it, though, considering how dumb you are.