Voltage difference supplied by a voltage (power) generator or a power distribution network, may fluctuate in its value. The fluctuations can occur due to short circuits, faults and overloads.
A device called a voltage regulator is used to regulate the supply voltage with in an acceptable range for house hold gadgets or other instruments.
If the input voltage (difference) is less than a calibrated value, the gadgets may not function. If it is more than a limit, then the gadget can get damaged. Hence, the input voltage is required to be maintained with in a range.
Thus the output of the voltage regulator is called regulated voltage. The regulated voltage device can supply a maximum rated current with in the range of voltages.