In the Declaration of Independence, we are told that all people have the inalienable rights to life, liberty, and the pursuit of happiness. But what do those things mean? Life is pretty obvious, but what, exactly, are liberty and the pursuit of happiness? This is what the Bill of Rights spells out.
In the Bill of Rights, we are told exactly what liberties American citizens have that are to be explicitly protected. We are told that we have the right to speak freely, to worship as we will, and to be free from governmental agents arresting us without a good reason (among other rights). By listing out these rights, the Bill of Rights is giving a more concrete definition of what our liberty and our pursuit of happiness (promised by the Declaration) includes.