What does this joke mean?

Cats have 9 lives (as the myth goes). That's it. Frogs make that "croaking" sound, so they "croak." To croak is another way of saying die. When someone "croaks," they die. So, if a frog croaks every night (they're always making that sound anyway), they "jokingly" die every night. And that's more than 9 times that they'll be coming back to life to croak again the next day! Therefore, according to the joke, frogs have more lives than cats. Hope I helped! :]