The final draft of the Constitution was signed on September 17, 1787 but did not officially become the supreme law of the United States until 9 of the 13 original states ratified, or approved, it. Some people thought that the Constitution did not protect some basic rights and, because of that, felt that it should be changed to protect those rights.
So Congress created a list of changes called amendments. These first 10 amendments are called the Bill of Rights and guarantee specific freedoms and rights.