ELI5: What is the deal with "Let's go Brandon"

Kiddo heard this on the radio today and now is asking me what that's all about. <br><br>I know enough that I can say it's about a phrase the president said and some people didn't like that phrase and now they saying "Let's go Brandon" in reference to that. <br><br>But I really want more context on this. It's all over the news, the radio and people are even saying it at sport games. I'm confused.