A constitution is the foundational law in most countries, including the United States. The term "constitutional provision" specifies that a rule or law comes from the constitution itself and not from statutory or common law.
The purpose of a constitutional provision is to establish the most basic vital rights, restrictions and organizations in society. Constitutional provisions establish broad notions of what is legal and what is illegal in a country, and they establish the structure of government.
Constitutional provisions cannot be altered or amended by legislatures or by the courts. The only way to amend a constitutional provision is to follow the procedures set out in the constitution itself, such as state ratification. The purpose of making it difficult to amend a constitution is to ensure that the constitution remains a relatively static document that protects fundamental and inalienable rights.
Constitutional provisions cover such topics as the rights and responsibilities of the president or governor, legislature and the court system. They also define the balance between state and federal governments.
Constitutional provisions also establish fundamental rights in society. For example, the Bill of Rights, the first 10 amendments to the U.S. Constitution, establishes such fundamental rights as freedom of speech, press and religion, the right to a trial by jury and the right to bear arms.
The U.S. Congress can pass laws that are consistent with constitutional provisions, but ultimately the U.S. Supreme Court is charged with interpreting and enforcing constitutional provisions. The Supreme Court's authority even includes the power to declare Congressional acts unconstitutional.