A well regulated militia, being necessary to the security of a free State, the right of the People to keep and bear arms shall not be infringed.
A militia is a citizen army, not a federally controlled military, or even a state controlled reserve unit. The militia can be considered any well bodied citizen that, at the drop of a flag can bear arms and defend their home and nation from any form of invasion, be it foreign or domestic. This type of citizen action is essential for the safety and security of a free state, so therefore the rights of the people to own and bear arms shall not be infringed. Plain and simple.
Having been oppressed by a professional army, the founding fathers of the United States had no use for establishing one of their own. Instead, they decided that an armed citizenry makes the best army of all. General George Washington created regulation for the aforementioned "well-regulated militia," which would consist of every able-bodied man in the country.