The due process of law is the legal requirement that provides protection to citizens, by guaranteeing respect to their legal rights, and preventing possible abuses performed on the individual by the law officers or the state. If a goverment is harming a person and is not following the predefined legal norms and regulations this would be considered a due process violation.
As part of the due process of law, individuals have the legal right of defending themselves in front of a court and to have an attorney, for example.