compliment
Explanation
The comic shows a man at a podium preparing to give a speech. In his internal monologue, he thinks: "I have to say something nice in my speech about him, but this guy is a moron. How can I say something that sounds complimentary, but which actually means that everything he says is either wrong or too vague to be right?" He then delivers his solution: "And so... he is the Nostradamus of his field!"
The joke is a perfectly constructed backhanded compliment. Nostradamus, the 16th-century French astrologer, is famous for his prophecies -- but those prophecies are notoriously vague, ambiguous, and only ever "confirmed" through extremely generous retroactive interpretation. Calling someone "the Nostradamus of his field" superficially sounds impressive and grandiose, but it actually means: this person makes pronouncements that are either wrong or so vague that they could mean anything, and people only think he was right after the fact by stretching the interpretation.
The humor works because it perfectly threads the needle the speaker set up for himself. The audience (both in the comic and reading it) can appreciate the elegance of a compliment that is simultaneously devastating. It also satirizes the culture of academic and professional speeches where speakers are often obligated to praise people they privately consider incompetent, leading to exactly this kind of artfully hollow praise. Anyone who has attended a conference or award ceremony will recognize the genre of compliment that sounds good but says nothing.