A White supremacist is an extreme racist, frequently a neofascist. White supremacists hate all other races and ethnicities and want to dominate them or push them to the brink of extinction. The Ku Klux Klan is one of the most notorious groups of white supremacists, as they spent decades attempting to force African Americans out of the United States for being a different color from theirs.
White Supremacists hate on other races simply because they have irrational fears of colors other than white. Some attempt to erase all color from the world. This is sometimes done in the name of their distorted Christianity.  The worst of them are as bad as Hitler. Others just want to drive non-whites out of places like North America where there are whites. They don't care that non-whites have been in America for generations. They don't care that the ancestors of the black Americans didn't choose to come there. They don't care if the black Americans have nowhere to go and no country will take them.
They always attempt poorly to defend their pathetic selves by spuriously claiming that whites invented "99.9 percent" of all civilizations, and insisting that other early, advanced, non-white civilizations such as China, Indus Valley, Maya, Inca, Olmec, Ancient Egypt, and Babylonia were either "primitive" or "were advanced only because whites gave them technology in the beginning". This is anything but true and in the case of China or Latin America utterly ridiculous. They also claim that paper was invented by a Frenchman, and not the Chinese or the Egyptians, (papyrus).
In addition, they claim that virtually all Native Americans are "savages" who "killed the first white colonists", when in fact the first Pilgrims were given enormous help by the natives, who taught them to grow corn and were even willing to share their resources; of course, the Pilgrims, being such kind people, killed the natives not long afterwards and stole their land. Remember the roots of Thanksgiving?
But worst of all, if you accuse a white supremacist of being a filthy bigot who tries to control the world under the claim that his skin makes him better than everyone else, then he will always claim without evidence that it's the Jews who are "supremacists" that "control the banks, the media, and the government". Hypocrites.
They also, to "justify" their hatred of Jews, claim that Jews aren't from the Caucasian group like they are. DNA shows that Jews have a little Non-Caucasian, but those same tests show that Whites>99% caucasian are rare. Just another example of how white supremacists use scapegoating to escape the fact that they are the ultimate scum of the Universe, not the darker-skinned and Semitic peoples around them.
They also spuriously claim that President Obama is "incapable" of running a country due to "genetic inferiority". The arguments they try to use just show that they are of low intelligence which may or may not be genetically based. Some white supremacists think that an ethnocentric white man like David Duke or Donald Trump is fit to be the U.S. president and mess things up for the whole world.
White supremacy is the root of a part of modern-day suffering, evil, stupidity, and deceit, along with conservatism, of course; but the two are frequently intertwined.
When white supremacist make stereotypical statements about other races, they themselves are perpetuating a stereotype about the white race. If black people see that a large group of white idiots are being racist, that reflects poorly upon the entire race. That could lead to a few of them having a hatred for decent white dudes and lead them to join a black supremacist group, like the Black Panthers.