Many Founders were worried that the rights of the people were not well protected by the new Constitution. They said they would not help to get the Constitution approved unless a bill of rights was added. The other Founders agreed to add one when the first Congress met, and that is exactly what happened.
The Bill of Rights is the name of the first ten amendments to the Constitution. This part of the Constitution was added in 1791. Since that time, other amendments have been added. Many of the amendments give rights to people who were not given these rights in the original Constitution. For example, the Nineteenth Amendment, added in 1920, gave women the right to vote.