Do you use natural gas in your home? Some people heat with gas. There are appliances that run on gas, also. Perhaps you have a gas stove. Water heaters and clothes dryers can also use natural gas.
If you use gas in your home, you'll have to pay for it. You will most likely get a gas bill every month. The gas company pipes the gas into your home so that you can use it. You have to pay for this service.
Like any other bill, you should take time to study your gas bill before you pay it. Check to see if your name and address are written correctly. Take note of the service period dates. Make sure that they gave you credit for paying last month's bill.