"A well regulated militia being nessesary to the security of a free state, the right of the people to keep and bear arms shall not be infringed." - Second Amendment of the U.S. Constitution.
It means we the people have the right to own and carry firearms, knives, swords or whatever weapons we want. The founders of this country believed that the people should possess the arms so they could defend their liberty from oppressive government. They were brilliant - they knew that government cannot be trusted. If only the government had weapons then they could push us around and there wouldn't be anything we could do about it. With arms we are citizens, without them we are subjects.
"Firearms stand next in importance to the Constitution itself. They are the American peoples liberty teeth and the keystone under independence." - George Washington
"No free man shall ever be debarred the use of arms. The strongest reason for people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government." - Thomas Jefferson
"Are we at last brought to such a humiliating and debasing degradation, that we cannot be trusted with arms for our own defense? Where is the difference between having our arms in our own possession and under our own direction, and having them under the management of Congress? If our defense be the real object of having those arms, in whose hands can they be trusted with more
propriety, or equal safety to us, as in our own hands?"
~Patrick Henry June 9, 1788, in the Virginia Convention
on the ratification of the Constitution.