What does a joke that is not a joke mean?

A joke that is not a joke means a cold joke. A cold joke means that the joke itself can't achieve the funny purpose because of boring, homophonic words, translation, or omitting the theme, different logic, judgment or special content, or because of the tone or expression of the performer.

The reason why cold jokes make people laugh is largely because the speaker's reasoning behavior is different from that of ordinary people in communication, and even makes it impossible for both parties to continue. When the listener's inference is inconsistent with the speaker's inference, the listener looks for his own mistakes, only to find that the speaker's inference is unreasonable and unconventional, with the help of homophonic or homophonic forms, or just the literal or natural meaning of the discourse.