"Let's Go Brandon" is a reporter's family-friendly interpretation of the "F*** Joe Biden" chant.
Brandon is Brandon Brown, a race car driver who won a race and gave an interview in front of a grandstand of people who were chanting "F*** Joe Biden!". The reporter reinterpreted it as "Let's Go Brandon!" and gave the world a clean way to say "F*** Joe Biden"
https://www.youtube.com/watch?v=TjsCOGaSjic