A well regulated militia made up of people who were supposed to bring their own guns and ammunition that they were proficient in using. The Militia Acts make this pretty clear, along with the Federalist Papers. The intent was that an armed population could be called on by the States to resist an invading army, be that army foreign or the standing Federal army. It also was an evolution of English law enshrining rights to self defense.
If we change the sentence slightly and say “The free flow of goods and services being essential to the safety and functionality of the economy, the right is the people to keep money and travel freely shall not be infringed”, would not imply that you are only free to leave your house and have cash if you are engaged in business.