Your monthly bill is made up of two parts; the Unit Rate (which is the amount paid for each unit of energy used), and the Standing Charge (which is a fixed daily charge).
It's easiest to think of the Standing Charge as a base rate; so this is the amount you get charged, even if you don’t use any energy. So, if your Standing charge is 20p per day, this will cost you £6 for the month.
The unit rate applies to the energy you actually use. So, if your unit rate is 12p per Kwh and you use 300 KwH then you will be charged £36.
So, the total of the charges for the unit rate and standing charge would be for the month would be £42.
A fixed tariff means, that your unit rate and standing charge will not increase over the duration of your contract. The only thing that will change (and increase or decrease your bill), is the actual amount of energy you use.