Substantive due process is a constitutional doctrine that protects certain fundamental rights from government interference, ensuring that laws and government actions do not infringe upon individual liberties without sufficient justification. This concept goes beyond mere procedural fairness and delves into the essence of rights, safeguarding vital personal freedoms such as privacy, marriage, and bodily autonomy. It connects to the broader notion of due process rights by emphasizing the importance of not just how legal proceedings are conducted, but also the substantive nature of the laws themselves.