Nascar is a sport where people drive cars in circles really fast. A reporter was interviewing a driver after he drove the fastest. The drivers name was Brandon. The crowd was chanting so loud it was louder than her interview. The reporter said that the crowd seemed very supportive because they were chanting "Let's go Brandon." The crowd was very obviously not chanting "Let's go Brandon." The crowd was chanting "Fuck Joe Biden." This reporter works for a news network that has a certain reputation for political bias. This event was seen as an obvious attempt to dissemble. This video went viral and had significant publicity.

One type of humor is known as "trolling". The basic idea is to see how much of a reaction you can get from something pointless and stupid. Now many people are making songs with the lyric "Let's go Brandon" and they are receiving hate mail, being banned, being demonitized, etc. This reaction is seen as outsized for a reference to a fairly well known piece of pop culture. Thus, to those who find outsized reactions funny, this is funny.

Humor is subjective

It must be funny, because many people find it to be funny.

Ours not to reason why.