An assassin threatens a planet.
The planet remains calm.
The assassin: "Do you not realize the gravity of this situation?"
Hey, you! Yeah, you, the one who still uses Internet Explorer. Listen up! This joke is funny because the assassin asks the planet if it understands the "gravity" of the situation. Gravity is a scientific thing that keeps stuff on planets, but it's also a word we use to mean how serious something is. The planet is all chill because, duh, it's a planet, so gravity is its thing!