Mega means million and a watt is a unit of power.
A megawatt hour is the amount of power used if 1,000,000 watts are used for 1 hour, or 1 watt is used for 1,000,000 hours. If 100 light bulbs each using 1,000 watts of power are turned on for 10 hours, they will use 100 x 1,000 x 10 watt hours = 1,000,000 watt hours = 1 megawatt hour.
Most electric companies charge for the number of kilowatt hours used. A megawatt hour is 1,000 kilowatt hours.