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.
Translation: In order for an independent free state to securely exist, the state must control a well regulated militia for defense.
Thus for the people to remain free of state tyranny, the people must be able to keep and bear arms in the event of said tyranny.
The militia is well defined by The Constitution:
Article 1, Section 8:
"The Congress shall have Power To [...] To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions; To provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress;"
Article 2, Section 2:
"The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States; [...]"