A toby is the water shut-off valve, generally located at the boundary of your property, that sits between the council water main and your private water pipe.
The toby allows the water to your house to be shut off - handy if you're having some plumbing work done or if your hot water cylinder springs a leak. Simply lift the cover and turn the handle of the toby until the water is turned off.
A toby may also be known as a stopcock or isolated valve.
The toby is usually under a black or blue plastic cover or under a metal cover - check out the examples below: