"Let's Go Brandon" is a phrase used by American right-wingers opposed to the 46th president of the
United States of America, Joseph R. Biden. It is code for "Fuck Joe Biden."
It started when a crowd at a NASCAR race shouted "Fuck Joe Biden" in protest of the current president. They claimed to be shouting "Let's Go Brandon" (Brandon being a
hypothetical NASCAR driver.)
P.S. Don't read the other
definitions. They're either politically biased or useless.