What did the supreme court mean by "separate but equal? why did many people feel this idea was faulty?

The "separate but equal" doctrine was ruled by the court case Plessy v. Ferguson. This doctrine meant that schools could be racially segregated as long as the educational opportunities remained "separate but equal". Many people thought the idea was faulty because African Americans were still discriminated against, and they continued to receive unequal treatment.
I hope this helped you!