The shut off valve is an oval shaped part attached to the main water line on the back of the toilet that turns the water supply on and off. The shut off valve works with the supply line and supply valve to bring water into the back of the toilet to fill both the tank and the bowl. If you notice issues with your toilet, one of the first things to do is turn the shut off valve to ensure no water is flowing into the toilet (to avoid flooding as other parts are removed). Once you have turned the valve as far as it will go, flush the toilet or lift the float lever. If the water supply is off, no new water will flow into the toilet. However, if the water line is still supplying water, the tank will refill when the flat is lifted.
A broken or busted shut off valve can complicate the process of turning off water supply to the toilet. If you do not have a functioning shut off valve, you will likely have to insert a screwdriver or use a pair of pliers to move the inner shut off pieces.