Jim Crow laws, which were enacted in the 1870s, brought legal racial segregation against black Americans residing in the American South. These laws were legally ended in 1964 by the Civil Rights Act of 1964.
Continued practices of expecting black people to ride in the back of buses or to step aside onto the street if not enough room was present for a white person and "separate but equal" facilities are instances of de facto segregation. The NAACP fought for the de jure law to be upheld and for de facto segregation practices to be abolished.
Public schools in any region of the US may be de facto racially segregated (or nearly so) simply because they are in neighbourhoods whose residents are all, or nearly all, of one race (such as urban ghettos or conversely, affluent suburbs).
This is opposed to de jure segregation, which prevailed in the American South and border states through the 1960s. Under de jure segregation, the law provided entirely separate schools for black and white students, which they legally had to attend, despite in many cases actually living closer to a school designated for the other race. In many cases, the schools for black students were older, had fewer resources of all kinds, and paid their teachers less than in white schools.