Fixed rate tariffs
This type of tariff guarantees the price of your energy for a set period of time (usually 12 to 18 months). After this you're free to switch and save again, but if you leave early, you'll probably be charged an exit fee.
Fixed rates are helpful for keeping control of your budget, and the rates you'll pay will probably be lower than those of the standard tariff. But remember what is fixed is the cost of each unit of power you use. If you start using more power than usual, your bills will go up but all the extra power used will be charged at the fixed rate. Fixed rate tariffs are designed to protect you from energy price rises.
When your fixed rate tariff ends you will need to find another fixed rate tariff or your provider will switch you over to a variable rate.