A frost free hydrant is basically an outdoor faucet with the valve buried far enough underground that it is below the frost line so it can't freeze. The exposed part is maybe 2-1/2' above ground. A lot of people would call them a faucet. The way they work is that when the valve is open (you raise the handle) water flows through but when the valve is closed a drain hole opens up so the part above ground can drain. No water, they don't freeze.
Alex I don't know what your safe depth would be. I have to bury that valve 24" to be safe from it freezing. Some people have to go a lot deeper, maybe as much as 4 or 5 feet below ground. It depends on how deep your ground freezes in your worst winter. Since they drain and don't freeze, I have running water outside all year around, handy for watering the chickens or cleaning anything I wish to clean.