Elsa: you can’t marry a man you just met

Elsa: you can’t marry a man you just met
Anna: ok boomer

This is an OK Boomer joke, which is a meme of sorts right now.

The phrase OK Boomer is used as a dismissive reply to the opinions of older people, so in this case, Anna is saying that she doesn't value the opinion of Elsa.

The listener to this joke is supposed to also recognize that the names Elsa and Anna are taken from the Disney Frozen movie, and this refers to a scene in the movie where Elsa probably said something similar if not identical to the setup.

The reason why it is funny is that Anna is being insulting to Elsa.