A number of studies have indicated a significant degree of unconscious race bias among Americans, particularly against African Americans. Experts in the fields of psychiatry, psychology, and neuroscience have been exploring a variety of possible remedial measures to counteract unconscious bias. Which is pretty damn exciting.
So, white friends, when your social justice oriented cousin is pissing you off by telling you that everyone is racist, including you, this is what they're talking about. They're often doing it inelegantly, inaccurately, completely counter-productively, and not uncommonly, with a big old heaping helping of *new* stereotypes about "white people." But this is the foundation on which they base their understanding that most - if not all - people have some degree of racial bias.