What is this in my garden?

[Rob]

Water shut off valve to my property? What are you supposed to use to turn it?

 

[Harvest Fields]

A square stop tap key?

 

[snowhead]

That's a Hydrant not a shut off for a property.
Same as you'd find in the street and as used by the Fire brigade.

Looks like this underground and will have a large water main connected to it, (or not, if it isn't connected up)
Underground Hydrant Valve

 

[ShaunCorbs]

You must live in the sticks a bit

 

[Meady]

Use it to wash your car, you will have plenty of pressure

 

[Ric2013]

I was going to say! Usually square top valves are not as modern as that, in my experience. The only one I've come across was in a council estate built in 1962-3.