It is the amount of electricity consumed if you have a one kilowatt appliance (eg a one bar electric fire) working for one hour. Your electricity bill is priced in so many pence for each unit, one unit being one kilowatt hour.
Carrying on from that, to work out the number of kilowatt hours (kWh) used, you multiply the kilowatts by the number of hours. So if you have a 2 bar fire on for 3 hrs, that would be 6 kWh, and so on.