It has become a very popular meme. Different versions of the "Let's go, brandon" song are now 1st, 2nd, and 3rd on iTunes.
The original is people at sports stadiums across the country have spent the last month chanting "Fuck Joe Biden!" They were doing this at a Nascar race and NBC Sports was trying to interview the winning driver, who's name was Brandon, and as soon as the cameras went love the entire, and I do mean ENTIRE grandstand started signing "Fuck Joe Biden" over and over and over. The partisan hack shill anchor woman at NBC Sports tried desperately to cover it up by claiming people were saying "Let's go, Brandon".
No one believed her and after Facebook, YouTube, and Twitter started banning people for posting "Fuck Joe Biden" people now say "Let's go, Brandon" to mean "Fuck Joe Biden". :)