To segregate is to separate from others. What this meant in Montgomery is that blacks could only sit in certain places in restaurants, that they could not go into certain businesses or that they could not use public rest rooms that were for "whites only."
While King was in jail, he wrote a letter to the newspaper explaining why he had broken the law. "I am here because injustice is here," he wrote. "I would agree with Saint Augustine that 'an unjust law is no law at all.'"