In Heller, the U.S. Supreme Court stated that "[t]he adjective 'well-regulated' implies nothing more than the imposition of proper discipline and training." (as in well regulated militia). Don't shoot the messenger.
What it means is that the Founding Fathers realized that the new nation needed a regulated military to defend itself. Having just won an armed rebellion against the king's troops, they realized that the average citizens needed the right to keep and bear arms as a deterrent against this regulated military being used against them by some unscrupulous leader in the future.
matt: It means you and other libs should read and appreciate the Constituion, the Dec. of Independence and other cherished documents and values this country was founded on instead of trying to tear them down like the pres.