The purpose of the Bill of Rights was to guarantee the individual rights of citizens under the Constitution. These are listed in the first 10 Amendments to the Constitution. Knowing the tendencies of governments to infringe on the rights of its citizens, many involved in writing the Constitution felt that these rights should be explicitly stated. In this way, the federal government could not arbitrarily abridge them.
The Bill of Rights restricts the powers and authority of the Federal Government and establishes many of the civil and political rights enjoyed in the United States, including the right to due process of law and the freedoms of speech, press, religion, assembly, and petition.
The Bill of Rights states what the government can and cannot do, and asserts the rights of the people, with which no law or government action should interfere.