Substantive due process is a constitutional doctrine that protects certain fundamental rights from government interference, ensuring that laws and regulations do not infringe on individual liberties without sufficient justification. This concept goes beyond mere procedural safeguards and examines the substance of laws to ensure they align with essential rights such as privacy, marriage, and family matters. It connects closely to the Bill of Rights by providing a framework through which courts evaluate the fairness and reasonableness of laws that may restrict these basic rights.