The meaning of "Well Regulated" ?

They could, individually, or in concert, "well regulate"themselves; that is, they could train to shoot accurately and to learn the basics of military tactics.
This interpretation is in keeping with English usage of the time, which included within the meaning of the verb "regulate" the concept of self-regulation or self-control (as it does still to this day). The concept that the people retained the right to self-regulate their local militia groups (or regulate themselves as individual militia members) is entirely consistent with the Framers' use of the indefinite article "a" in the phrase "A well regulated Militia."