It’s the war flag of the confederates, the group that very openly stated their goal was “states rights” to choose slavery.

The flag that’s generally presented as the Confederate flag is actually not the Battle Flag; it’s the “Dixie”, which has the same general design as the Battle Flag but is a different shape (i.e. rectangular instead of square). And yes, the Confederates were fighting for states’ rights, which included the right to hold slaves. I don’t agree with that part of their platform, But I also disagree with the series of violent and/or unconstitutional acts that ostensibly reunited the nation.