Back in the day “well regulated” meant “well armed and equipped”.

So, in order to form a proper defense of the country, any able bodied man could be called up (the militia), and it was necessary this body of men be well armed and equipped.

You may already know this, but Federalist Paper No. 29 touches on that subject.

"The project of disciplining all the militia of the United States is as futile as it would be injurious, if it were capable of being carried into execution. A tolerable expertness in military movements is a business that requires time and practice. It is not a day, or even a week, that will suffice for the attainment of it. To oblige the great body of the yeomanry, and of the other classes of the citizens, to be under arms for the purpose of going through military exercises and evolutions, as often as might be necessary to acquire the degree of perfection which would entitle them to the character of a well-regulated militia, would be a real grievance to the people, and a serious public inconvenience and loss. It would form an annual deduction from the productive labor of the country, to an amount which, calculating upon the present numbers of the people, would not fall far short of the whole expense of the civil establishments of all the States. To attempt a thing which would abridge the mass of labor and industry to so considerable an extent, would be unwise: and the experiment, if made, could not succeed, because it would not long be endured. Little more can reasonably be aimed at, with respect to the people at large, than to have them properly armed and equipped; and in order to see that this be not neglected, it will be necessary to assemble them once or twice in the course of a year.

https://avalon.law.yale.edu/18th_century/fed29.asp