A more thorough explanation:

The separate-but-equal doctrine was a legal rule that allowed for the segregation of African-Americans as long as they were given equal opportunities and facilities in education, public transportation, and jobs. This rule was established in the Plessy v. Ferguson case in 1896.

For example, under this doctrine, schools could be segregated as long as both white and black students had access to the same resources and facilities. However, in reality, the facilities provided to African-American students were often inferior to those provided to white students.

The separate-but-equal doctrine was overturned in the landmark case Brown v. Board of Education in 1954. The Supreme Court ruled that segregation in public schools was unconstitutional, as it violated the principle of equal protection under the law guaranteed by the 14th Amendment of the U.S. Constitution.