multiverse
Explanation
The Joke
A man is being confronted (presumably by his boss) about something he has done wrong at work -- most likely he is the one not wearing pants. His retort is to invoke multiverse theory: "Oh yeah?! Well there's an infinite number of universes out there where I am firing YOU for not wearing pants to work!" The caption below reads: "Multiverse theory has really improved my life outlook."
The man is clearly in the wrong and in a position of weakness, but he takes comfort in the idea that somewhere in the infinite multiverse, the situation is reversed and he is the one in charge, firing his boss for the same offense. He has essentially used the concept of infinite parallel universes as a coping mechanism for his own failures.
The Humor
The humor comes from the absurd misapplication of a real physics concept as a self-help tool. The multiverse interpretation of quantum mechanics suggests that every possible outcome is realized in some branch of reality. The man has taken this to its logical (but ridiculous) conclusion: no matter how badly things are going for him, he can console himself with the knowledge that in some universe, he is doing great. The fact that these alternate versions of him are entirely separate and inaccessible does not diminish his comfort.
The joke also plays on the human tendency to rationalize and find consolation in irrelevant facts. The man is not actually better off because of the multiverse -- he is still pantsless and about to be fired in this universe -- but he has found a way to feel empowered about it. It satirizes how people sometimes misuse scientific concepts to justify comforting but meaningless conclusions.
References
The comic references the many-worlds interpretation (MWI) of quantum mechanics, proposed by Hugh Everett III in 1957, which suggests that all possible outcomes of quantum measurements are realized in separate, branching universes. In popular culture, this has been simplified to the idea that "every possible universe exists," which is the version the character is invoking.