Racialism is the idea that humanity can be easily divided into well-defined categories ("races") that are both broad (each category should include many humans, such as entire continents' populations) and clearly-defined (the categorization method should rarely misidentify someone's "race"). Racialism implies that these races are substantially different from each other and that these racial differences strongly determine the abilities and behavior of individuals and peoples. Essentially, racialism argues that human populations are substantially different from each other to a degree which necessitates biological classification below the species level. In short, racialism holds that biology divides humans.
Very few racialists call themselves "racialist". Their preferred labels include scientific racism (pre—2000), race realism or racial realism (post—2000), and human biodiversity or HBD (post—2010). Racialism was first developed in the 1700s. It remained virtually unchallenged until the 1930s-1960s, when genetics showed it to be erroneous. Simply put, modern racialism is pseudoscientific bullshit.