While I do not disagree with anything that is said in the previous answer, I think that there is something that should be added here.
The first answer does an excellent job of laying out the classic answer to this question. The Bill of Rights (it says) lay out the rights that we enjoy as Americans. The legal system gives us a means by which to enforce those rights. This is not wrong.
However, we can look at this question in a deeper way. We can say that the mere existence of the Bill of Rights and the legal system do not really protect our personal freedom. If they did, there would not be as many instances of rights being trampled as there have been in our history. If they did, African Americans would not have been denied rights for decades. Anti-war protestors would not have been jailed in WWI. Americans of Japanese descent would not have been interned in WWII. Communists would not have had their freedom of speech and association restricted during the “Red Scare.”
What this tells us is that our rights are only truly protected when we stand up and fight for them. The Bill of Rights on its own cannot protect our rights. Neither can the legal system. It is “we the people” who protect our own rights. Rights are only protected by political action on the part of citizens.
So what is the role of the Bill of Rights and the legal system? Eugene Rostow once said that the justices of the Supreme Court were “teachers in a vital national seminar” about what democracy means and what our rights should be. This is, in one view, what the Bill of Rights and our legal system do. They teach us what our rights should be. They encourage us to believe that we have those rights. They encourage us to believe that others have those rights. When we are convinced that these rights are important, we stand up and fight for them.
The Bill of Rights and our legal system cannot protect our rights if we do not stand up for them or if too many people oppose those rights. Their main function is to inspire us to act to protect rights that we believe in.