"Liberty" is a word meaning freedom. In a political sense, the word usually refers to being free of unjust or tyrannical political control.
An "amendment" is simply a change (usually for the better), especially in reference to documents and records. The U.S. Constitution famously lists 27 numbered amendments. These amendments are featured at the end of the constitution, and serve as important modifications or additions to the rules and rights of Americans and their goverment, as written in the original document.
A "bill of rights" is a statement of basic rights believed to extend to all citizens of a certain society. They are usually outlined in the official governing documents of a particular country. In the United States, the Bill of Rights is considered to be the first 10 amendments of the constitution.