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Explanation
The Joke
The comic is presented as a single panel addressed to "Literature students." It offers advice for the next time they have to review a book they have only skimmed: use a quote from Jorge Luis Borges reviewing James Joyce's "Ulysses." The Borges quote reads: "I confess that I have not cleared a path through all 200 pages of it. I have merely examined only bits and pieces, and yet I know what it is, bold and legitimate and copious and altogether too intelligible, certainly, without ever having been read from the first word, constituting virtually a city without having been to many of its streets." The source is cited as Borges' "Selected Non-Fiction," edited by Eliot Weinberger.
The joke is that Borges -- one of the most respected literary figures of the 20th century -- essentially admitted to not fully reading Joyce's "Ulysses" and still wrote a confident, eloquent review of it. The comic suggests that students who have only skimmed a book can cite this passage as a defense, since even Borges did the same thing.
The Humor
The humor works because it validates a universal student experience (not finishing an assigned book) by invoking the authority of a literary giant. Borges' quote is genuinely beautiful and erudite, yet its content is essentially a sophisticated version of "I didn't read the whole thing but I get the gist." The irony is that students could use this quote in a book report about not reading a book -- itself a form of literary commentary they did not fully engage with. Weiner is also poking fun at the literary establishment, where sufficiently eloquent prose can make even an admission of laziness sound profound.
References
Jorge Luis Borges (1899-1986) was an Argentine short-story writer, essayist, and poet, widely regarded as one of the most important literary figures of the 20th century. James Joyce's "Ulysses" (1922) is considered one of the most important -- and most notoriously difficult -- novels in the English language. The quote is from "Selected Non-Fictions" (1999), edited by Eliot Weinberger.