A fixed tariff means that the rates we charge for each unit of energy are fixed for a set period. It doesn’t mean that you will pay a guaranteed, fixed amount every month for the entire contract. That of course, depends on how many units you use.
If you fix your energy rates you know that even if the wholesale cost of energy goes up, your rates will stay the same until the end of the contract. Of course, if prices go down, your rates will remain the same.