Welbeck. A fixed tariff is just that - the cost per unit of gas or electricity is fixed for the term of your contract.
The advantage is that should there be an increase in the cost, it won’t affect you. Yes, it’s a gamble, but I think it’s safe to say prices usually rise. I re-signed with Octopus on a fixed rate contract in August - the unit prices were lower than the previous year, although the standing charges had risen.
You are perhaps getting confused with a fixed monthly payment as ‘suggested’ by the energy provider, but that can be challenged and changed.
I actually prefer to pay the same every month and build up a bit of credit in summer towards the increased winter bills.