Your energy bill is made up of two elements; a standing charge and a usage charge.
Firstly, the standing charge. This is the amount you pay regardless of how much energy you use. This covers the cost for of the supplier maintaining the energy network, your meter, engineers etc.
Secondly, your usage charge. This is calculated based on the amount of gas and/or electricity you consume. This is typically measured in kilowatt hours.
There are two main types of energy plan - a fixed plan and a variable plan. If you have a variable plan, then your bill will vary each month based on the standing charge and how much you used in the prior month. If you have a variable tariff, the rates can also change leading you to pay more the same amount of usage.
If you have a fixed plan, then the rates are fixed for the period of that deal, typically 12 to 24 months. If you have a fixed deal, your energy supplier will estimate your annual usage and set a monthly fee that should stay fixed each month. They will still calculate your actual usage, and at the end of your fixed deal you may have a positive or negative balance, depending on whether your usage was higher or lower than their estimate. If your usage changes significantly during your fixed deal, they may suggest changing your monthly payments to make sure you aren't building up a balance you would need to pay off at the end of the deal.