No Soldier shall, in time of peace be quartered in any house, without the consent of the Owner, nor in time of war, but in a manner to be prescribed by law.
Basically: You cant just dump your armed forces off on the citizenry, you have to provision them properly. In peacetime, that means you have to own up to keeping an active military yourself, stupid aristocrats. In war time, the only support you can demand of this type is what actually has laws on the books to govern such support. Anything not explicitly mentioned in that law is not required, and should not be expected.
It was a serious problem for the ruling elite to raise up big standing armies to go "raw raw, our penis is the biggest!" to each other with (see for instance, some of the bullshit from NK) while failing to adequately feed, clothe, house, or otherwise see to the needs of that army. Instead, the citizenry was EXPECTED to make up for that shortcoming, because they were subjects of the crown, and thus essentially the king's property (and if he wills that you quarter half a battalion in your basement, then by jingo make it so!) All too often, this led to serious misconduct from said standing armies, because they could demand anything, and the people had no grounds to refuse. (this included such nasty things as rape.)
This little thing is kinda a big deal. Imagine if Uncy Sam decided "Hey, you know that multi-trillion dollar war machine we shovel money into each year? Yeah-- let's force ordinary Americans to warehouse, feed, clothe, and otherwise pay for everything straight up, or they can go to jail. In fact, let's couple that with that fancy civil forfeiture stuff we like so much. Yeah, that's awesome-- GO US for saving the government money!"
Because as absurd as that sounds, there would be SOMEBODY who thinks that would be a swell idea.