Water. Notice the threads are standard hose bib threads (forget what they’re called, it’s not pipe thread, just similar).

It uses a key to operate it, helps prevent average jerks from just turning it on. (These keys are widely available for about $5)

Edit: the little cap is for freeze protection. It’s part of a mechanism to vent water from the last foot of the pipe and bib, so there’s no water in it to freeze when the valve is closed.

helps prevent average jerks from just turning it on. (These keys are widely available for about $5)

So only above average jerks can turn it on?

Or below average…

We call ourselves the Five Buck F**ks.