Inside the tank is a water valve that controls the water for the refill cycle. When the toilet is flushed, the water is released from the tank and the float sitting on top of the water, drops down, engaging the tank valve to release water into the tank.
If the tank float is misadjusted the toilet may never stop filling, it may under-fill resulting in insufficient water to properly flush the bowl, or you may be simply wasting water by using more water than is necessary.
To adjust the tank float, you simply change the height at which it causes the water valve to shut off. By increasing the height of the float, more water will fill the tank. Lowering the float reduces the water level.
Remove the lid from the tank and set it carefully onto a towel. Vitreous china can easily scratch other surfaces, so a towel is highly recommended.