After the US Constitution was first written and signed, it had to go to state conventions in order to be ratified. To be ratified, it had to be approved by at least three-quarters of the states. In many of these states, there was a lot of hesitancy about ratifying this document, because it did not include any protections of freedoms and rights.
At these state conventions, there were those who suggested changes, or amendments, to the Constitution before they would approve it. They wanted these amendments to protect both citizens and the states from the possibility of a tyrannical or overbearing federal government.
Some people at these state conventions wanted to make changes that amended the actual structure of the federal government. Even more than this, however, there were many who wanted to have a list of protected rights added to the Constitution. Many state constitutions already included fundamentally protected rights, and they wanted the federal government to do the same. This list of rights became the first ten amendments to the Constitution and is commonly known as The Bill of Rights.
It was clear during these state conventions that the Constitution stood little chance of being ratified without a Bill of Rights. James Madison, one of the biggest proponents of the new constitution, understood this. He went to several state conventions to make it clear that a list of rights would be included. Madison took the best ideas from these various state conventions and distilled them down into what would eventually become the first ten amendments.