Because on the one hand it's simple, the other it's not. It's been covered in here relentlessly over the last week, also it's been covered relentlessly in the news for longer, but especially the last week since Trump's comments. Just go back a few pages in this thread to last Saturday/Sunday and you will be able to read as much about it as you like.
Basically a black NFL player called Colin Kaepernick started sitting out the national anthem to protest police brutality and the murder of black people by cops. He was told it would be more respectful to kneel instead, so he did. He lost his job and is unemployed but still protesting and a few other NFL players started kneeling in support of him and his protest. Trump called them out and said it was unpatriotic and disrespectful to the flag and the USA and basically made the argument all about him. Last week over 150 players, owners and managers kneeled or took part in a joint protest against police brutality but also now against Trump himself. This caused a massive split in the USA with some supporting and some against.