A reporter was interviewing a race car driver named Brandon at a track following his win in a race. The crowd was chanting a slogan that the reporter mistook to be "Let's Go Brandon", which would make sense, right? It's a race, and Brandon just won? However, for some unknown reason, republicans/rednecks/heathens have decided it makes sense to chant "Fuck Joe Biden" in situations like that. So after this, "Let's Go Brandon" became something degenerates would put on flags, bumper stickers and coffee mugs and so on, as a stand in for "Fuck Joe Biden", demonstrating their high levels of class. Then, someone online made images of Biden looking sorta evil with glowing eyes etc, which were posted in actual appreciation of Biden, sort of making fun of the right-wing claims that he's incompetent and senile. You could read this article for a more thorough explanation plus some examples.