A kWh is also known as a kilowatt-hour. This is the unit your energy is measured in. It measures the amount of gas and electricity that you are using. It refers to the use of power over a period of time e.g. a 1kW drill used for an hour will use 1kWh. The amount you pay per kWh varies between suppliers but you can see how you are charged on your bill.
You can find out about your kWh by looking on your bill, annual statement or even on your smart meter (if you have one installed). On your bill it will break it down for you into sections, such as kWh and standing charges etc.