Two things that are defined by racism at their core are the Ku Klux Klan (KKK) and the Jim Crow Laws.
The KKK was started after the civil war to keep black people from voting for Republicans. It had a resurgence in the early 20th Century due to the efforts of President Woodrow Wilson. While they seemed to hate pretty much everyone that wasn’t like them, they had particular ire for black people. There did a lot of physical damage in the form of terrorism, including lynching, bombing, and beating.
The Jim Crow Laws were a political movement of forced segregation, which used the expression “separate but equal” as a justification of the poor treatment of minorities. Businesses would get in trouble for allowing races to eat together, schools weren’t allowed to teach mixed races, and all facilities were seperated.