Simple Definition:

A watt hour is a unit of energy (the amount of power used over a period of time) that measures the amount of electricity consumed by a device with a power rating of one watt, in one hour. [Power rating refers to the amount of power a device uses. Electricity is the flow of electric charge.]

For example, if a device with a power rating of 10 watts is used for 5 hours, it would consume 50 watt hours of electricity.

Very Simple Definition:

A watt hour measures how much electricity a device uses. It tells you how much power a device with a power rating of one watt uses in one hour.

For example, if a light bulb that uses very little power is turned on for an hour, it would use one watt hour of electricity.