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.

The Bill of Rights is comprised of the first ten amendments to the United States Constitution.  The original draft of the Constitution passed at the Constitutional Convention in Philadelphia, Pennsylvania in 1787 was an incomplete document that was concerned primarily with establishing the instruments and structure of the federal government.  As the designation of “amendments” implies, they were added to the text of the Constitution in the years following the original document’s adoption at that convention, with Amendment I adopted in December 1791.  While the amendments comprising the Bill of Rights were added to the original Constitution over time, their importance to the preservation of civil liberties cannot be overstated.  Some of these amendments, especially the first, second, eighth, and tenth amendments, are routinely subject to debate regarding their proper interpretation.  Combined, however, they have provided the basis for the freedoms that made the United States a sanctuary for millions of people fleeing persecution from all over the world.

The rights ensured in the Constitution are precisely that: rights.  They are not privileges American citizens enjoy at the behest of an autocratic government.  They are rights established by their authors and, in many instances, reaffirmed in decisions rendered over the decades by the U.S. Supreme Court.  Which brings us to the second part of the question: the role of the legal system in preserving liberties.  Americans enjoy the right to petition their government for redress both through direct appeals to their representatives in government and through the civil justice system that allows for redress of grievances through the filing of suits in courts of law.  While neither avenue promises successful resolution of disputes from all perspectives – that would be a physical impossibility – their existence and availability ensures that all citizens have legitimate avenues through which to air their grievances and seek redress.  Admittedly, the financial costs associated with recourse to the civil justice system can be prohibitively expensive, especially when confronting large institutions with essentially unlimited resources.  The possibility and option of pursuing that avenue nevertheless exists for those willing to see the pursuit of justice through to its conclusion.

These are the fundamental roles of the Bill of Rights and the legal system in protecting the liberties American citizens enjoy.  Ongoing debates regarding issues like gun control, freedom of speech when vitriolic pronunciations can inflame passions, and privacy from unreasonable searches remind us daily of the fragility of these rights during difficult times.  Their continuing importance, however, cannot be denied.