It means exactly what it says, that the people’s individual right to be armed will be respected and that the resulting armed populace will be secure against tyranny, invasion, and crime. Our founding fathers were careful to construct this sentence in such a manner that there could be no other reasonable interpretation besides the obvious.
History is littered with stories of unarmed citizens being taken over by dictators (Germany is one such example). Could that happen here? Not if we maintain the right to keep and bear arms. No other country has the freedoms that we have here in the United States. This is truly the most awesome place in the world to live, and I would like to keep it that way.