It means 'fuck Joe biden', there was some NASCAR thing where people were chanting 'fuck Joe Biden'. It was being reported on and the reporter relayed it as 'lets go brandon' and it became a meme on the right.

The meme in this thread is about how American conservatives completely miss the nuance of why people on the left actually oppose both them and centrist Democrats. It's easier to do childish shit and just assume that everyone to their left is some form of liberal who loves Joe Biden. The good news is they're not getting younger.