xkcdexplained:

A man (presumably an artist based on his hat) and a female enjoy the night’s stars together. They discuss a scientific fact about star light, then the artist then fetches a frame and holds it up above him while looking through it. He did this because he believes that the night’s sky is art, and people often frame art before viewing it.

Later she will probably talk about astronomy while he does oral sex on her.

Not to be pro-XKCD or anything, Randall Munroe is an asshole, but you literally did not ‘get it’. He’s not holding up a frame, he’s holding up a mirror. The joke is that he’s reflecting the light back so that they can go home instead of dying in his eyes. The hat is not to indicate that he’s an artist, it’s to indicate that he is a recurring character whose only character trait is his naïveté.