Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

This comic parodies the Superman trope of Clark Kent ducking into a telephone booth to change into his superhero costume, but with a capitalist twist. When aliens attack the Metropolis Stock Exchange, the bespectacled man in the first panel doesn't rush to save the day as a superhero -- he rushes to a telephone booth to call his stockbroker.

The punchline, "Sell! Sell! Sell!" replaces the expected heroic transformation with pure financial self-interest. Rather than being concerned about the destruction and danger to human life, the character's first instinct is to protect his investment portfolio before the market crashes. The comic plays on the visual setup familiar from Superman stories to deliver a joke about how financial anxiety can override even the most dramatic real-world events.