The Bill of Rights refers to the first ten Amendments to the Constitution, which set out individual rights and liberties. Though these rights originally only applied to the federal government, through the legal doctrine of incorporation, most of the Bill of Rights is now applied to the states through the Due Process Clause of the Fourteenth Amendment.
See e.g., the First Amendment as an example.
[Last updated in July of 2022 by the Wex Definitions Team]