I think it was for the BLM social movement, but that was way back in either 2016 or 2018. Anyways, he was the first football player to kneel during the national anthem, but he got a TON of backlash for doing so (the NFL's main audience is usually middle-aged men, many of which have served our country, so this really did not make them happy when Kaepernick pulled this stunt). Now, keep in mind, it wasn;t just the people- the NFL has INCREDIBLY strict rules of what both cheerleaders and football players can and can't do, wear, say, eat, etc. It's basically like a super strict modelling agency! I think what he did also violated the NFL's code of conduct. He eventually got let go from the team, and has been trying to get back into the NFL ever since, but with no luck.
So I suppose you could say it brought awareness to the BLM movement, but all it really did was get him (possibly) kicked off the football team 
Many others would kneel after Kaepernick, but I think they got in trouble as well, and I believe since then it is very rare to see a NFL player kneel during the anthem.
