meaning
Explanation
The Joke
A person arrives in the afterlife and asks God the big question: "Why am I here? What's the meaning of it all?" God begins to answer -- "Well, I can tell you, but how would you know whether this is really God and not just a hallucination?" The person screams in frustration ("AAAAAHHHH!"), and God quips, "God, you're mean."
The comic takes the classic scenario of meeting God after death and asking for the meaning of life, then undermines it with an epistemological problem. Even if God gives you the answer, you have no way to verify that you are actually talking to God rather than experiencing a dying brain's hallucination. The answer to life's ultimate question is rendered worthless by the inability to trust the source.
The Humor
The humor lies in the cruel irony of the situation: you finally get to the one being who can definitively answer your deepest existential question, and then immediately realize you can never be sure the answer is legitimate. It is a compact philosophical joke about the problem of verification and epistemic certainty. God's final pun -- "God, you're mean" -- adds a light touch, using the divine name as a casual expletive, which is itself a fun irony given the speaker.