Electrical equipment has a design operating point. The data for this (volts, amperes, frequency) and other things such shaft output power and speed, for example, for a rotating machine help define the operating envelope.
RATED voltage is the voltage on the nameplate - the "design point" for maximum power throughput and safe thermal operation. OPERATING voltage is the actual voltage applied at the terminals to the electrical equipment.
For example - a machine has a rated voltage of 4160 volts at 60 Hz. However, due to other loads on the utility and the location of the facility distribution bus, the operating voltage applied to the motor terminals is only 4053 volts at 60.1 Hz.
Important to note - the operating voltage is not always lower than the rated voltage. The facility could be using fixed power factor correction; when the bus is lightly loaded, the machine could see 4280 volts at 59.8 Hz, for instance.