They want to put racial minorities back into a permanent second-class citizenship. They want to restore segregation and discrimination back into the Southern states. (They also hate foreigners and immigrants, Jews, Catholics, gays, feminists, etc. etc. but for the moment it's more about race.) White Supremacists see the US as a white/male/Christian country and everyone else should take a back seat.
As others here have pointed out, the US is losing its white majority. This has a lot of white people totally freaked out, thinking that as a minority they will be mistreated as badly as THEY have mistreated minorities. Trump and the Republicans have promised to stop this trend, but they really can't.