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Explanation
In the first panel (labeled "Earlier"), a concertgoer raises their hand and asks an innocent question: "Knowing evil Paul Simon bad music, what do you love?" (or similar provocative musical opinion). The second panel jumps forward in time to show the aftermath: a crowd of people passionately arguing and shouting "NOOOOOOOOOOOOOOO" -- someone is exclaiming "Oh my God! My God! He's just a people-talking-about-how-much-they-hate-bad-music-and-how-it-lasts-forever-and-ever-and-ever" (or words to that effect about how musical debates spiral out of control).
The joke plays on how expressing a strong or controversial music opinion in a group setting can instantly ignite an enormous, never-ending argument. The humor mechanism is the contrast between the casual, almost offhand way the question is posed (one person calmly raising a finger) and the apocalyptic reaction it triggers. This is a classic SMBC structure where a small, innocuous action leads to absurdly outsized consequences.
The comic satirizes music fandom culture and the way people become intensely passionate -- even combative -- about their musical tastes. It suggests that asking someone about music they dislike is essentially throwing a grenade into a social gathering.