The following is not my wording, but lifted from a piece on the subject. I think it makes it pretty clear:
What is a militia?
At the time of the American Revolutionary War, militias were groups of able-bodied men who protected their towns, colonies, and eventually states. “[When the Constitution was drafted], the militia was a state-based institution,” says Rakove. “States were responsible for organizing this.
“What did it mean to be well regulated?
One of the biggest challenges in interpreting a centuries-old document is that the meanings of words change or diverge.
“Well-regulated in the 18th century tended to be something like well-organized, well-armed, well-disciplined,” says Rakove. “It didn’t mean ‘regulation’ in the sense that we use it now, in that it’s not about the regulatory state. There’s been nuance there. It means the militia was in an effective shape to fight.”
In other words, it didn’t mean the state was controlling the militia in a certain way, but rather that the militia was prepared to do its duty.
Good post.