After the Civil War, African-Americans were emancipated from slavery in the entire United States. However, many states subsequently passed laws requiring the separation of "people of color" from whites. These "Jim Crow" laws policed everyday life, and made it so black people and white people could not go to the same schools, sit in the same section of movie theaters, ride in the same sections on buses, or even drink from the same water fountains.
A man named Homer Plessy attempting to sit in the "white section" of a bus led to the landmark Supreme Court case Plessy vs. Ferguson which stated on the one hand, that no state could make a law that limits the privileges or rights of a US citizen, but on the other hand, that so long as segregation is "separate but equal," it is legal. Thus, in the North and the South, segregation continued into the 20th century, with situations that were certainly not "equal" being largely overlooked by the government. Though racism was less violent in the North, most neighborhoods were racially segregated, leading to segregation in schools, restaurants, churches, and more.
The 1950s and 1960s were times of activism both for educated blacks taking their first major government positions (for example, Thurgood Marshall, a black lawyer and later Supreme Court judge, who argued in Brown vs. Board of Education that there was no such thing as "separate but equal" schools) and the grassroots activism of the Civil Rights Movement. However, though few would actively advocate for segregation today, throughout the 20th century and even into the 21st century, many areas throughout the United States remain racially segregated (an issue perpetuated by the wealth divide that remains between blacks and whites).