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Explanation
This comic is a linguistics joke about prepositions. One character says "And anyway, that's what I'm thinking of at at at at at." The other character is baffled: "What... what was that?" The first character calmly replies: "Good grammar."
The caption below delivers the punchline as a "Language Pro Tip": "There's no rule against ending your sentence with six prepositions."
The humor plays on the old (and widely debunked) grammar rule that you should never end a sentence with a preposition. This supposed rule has been a source of pedantic correction for centuries, even though it was never a genuine rule of English grammar -- it was borrowed from Latin grammar and awkwardly imposed on English. The comic takes the reductio ad absurdum approach: if ending a sentence with one preposition is supposedly bad, what about ending it with six? The joke is that you technically can construct a grammatically valid (if absurd-sounding) sentence that ends with a string of prepositions.
The sentence likely describes a nested series of topics or locations, where each "at" refers to a different layer of what's being thought about. It sounds completely ridiculous when spoken aloud, but is technically parseable English. The comic celebrates the delightful weirdness of the English language and mocks prescriptivist grammar rules by showing how far you can push them.