Let's Do This
Explanation
The Joke
A woman named Sally is asked by her partner if she ever feels like none of what they do matters. Her partner responds with an impassioned, motivational speech: "All of it matters! This place, this moment, you, me. Let's own it! Let's rock this thing!" It is a classic carpe diem moment -- seize the day, live in the present, nothing is meaningless.
The caption then delivers the punchline: "Five billion years later, the Sun expanded and dimmed, cremating the inner planets and freezing the outer planets, extinguishing the evidence of past life and the possibility of future life." The comic brutally undercuts the motivational speech by zooming out to the ultimate cosmic timescale, where everything is indeed rendered completely meaningless by the inevitable death of the solar system.
The Humor
The humor is in the savage juxtaposition between the heartfelt, inspirational "let's rock this thing!" attitude and the cold, clinical description of the Sun destroying all evidence that any of it ever happened. It is a classic SMBC move -- using cosmological timescales to deflate human pretensions. The comic does not necessarily argue that nihilism is correct, but it does point out the tension between our need to find meaning in the present moment and the universe's total indifference to whether we do or not. The deadpan scientific language of the caption makes the devastation even funnier.