MWh is an abbreviation of megawatt-hour. It is a unit of energy production or usage. Mega = million, so one MWh is equivalent to one million watts produced/consumed over one hour. Most commercial power plants are described in terms of their potential power output over one hour in MWh. Residential solar installations and the like are more likely to be described in KWh.
1000 watt-hours = 1 kilowatt-hour (KWh)
1000 kilowatt-hours = 1 megawatt-hour (MWh)
1000 megawatt-hours = 1 gigawatt-hour (GWh)
1000 gigawatt-hours = 1 terawatt-hour (TWh)