It means that you've missed out on a joke because you didn't have the necessary knowledge to realise that it was funny.
One of my friends, a doctor at a Sydney hospital, saw that a patient had been re-admitted for the third time suffering the same life-threatening symptoms. Each previous time he had been treated and sent away with prescriptions for a course of medication that would have had him back to his normal health.
Unfortunately he was an alcoholic and spent his money on booze rather than the prescription.
When my friend checked his status sheet he saw that the admitting doctor had diagnosed him as suffering from neuronepaenia.
That's an in joke and would only be funny to anybody who knew that, in medicine, "paenia" means there is something missing. So neuronepaenia? No neurones equals no brains.