What was the significance of the Supreme Court's ruling in Plessy v. Ferguson?
It set the stage for the future of the American civil rights movement.
Homer Plessy's ethnicity was rather ambiguous; he was white by appearance, but genealogically he was part African American. Therefore, when he boarded a "white-only" car in Louisiana, he was rejected, and punished. Plessy took his issue to the regional courts, then the state courts, then finally the Supreme Court. In the Supreme Court, a majority decision deemed Plessy's argument invalid.
Thus began the "separate but equal" doctrine. By ruling in favor of segregationists in the case, the court gave the nation the justification to segregate the colored Americans from the White Americans.
This would invoke a new issue for American minorities to fight against; the rampant segregation that emerged in the United States. For the next 50 years, in the midst of global crisis and depression, minorities would fight for their rights.
By 1954, the court case Brown v. Board of Education would deem segregation unconstitutional, and a series of acts in the 1960s, would end segregation.
So, Plessy v. Ferguson was a landmark case in U.S. history, that changed the motives and energies of the civil rights movement.