Segregation means to separate groups of people based on race. Segregation can be forced or self-imposed.
In the southern United States, the policy was created by government mandates at the behest of the Democratic party starting in the 1870s. The Democratic party has been known ever since as the party of segregation. Segregation was allowed in public schools by the U.S. Supreme Court in Plessy v. Ferguson (1896), but then prohibited in Brown v. Board of Education (1954). In the 1960s, "Segregationists wanted policies that privileged whites."
In addition to the public schools, segregation existed in the United States in the military before 1950. In the South it existed in public accommodations like trains, buses, restaurants, movie theaters and hotels before being banned by federal law in the Civil Rights Act of 1964.