What does he mean by "good one"?

Native speaker’s answer
Rebecca
SpongeBob thinks Squidward is joking when he says he doesn't like crabby patties, so he responds with "good one"! It's something people say when someone tells a funny or good joke. "One" refers to the joke that was told. Ex: Good one! That was funny. Ex: You want me to tell a joke? Sorry, I don't know any good ones.