The unconscious biases people walk around with in their heads. They're things people pick up without realizing it that lead to friction for minorities in society.

Studies have shown that people who don't think themselves to be actually racist still act in race-discriminatory ways. For example, people with non-ethic-names get more callbacks for their resume controlled for qualifications on the resume (https://www.nber.org/digest/sep03/employers-replies-racial-n...).