An electricity tariff refers to how you are charged for the amount of electricity that you use. Depending on your energy provider and where you live , there may be different types of electricity tariffs.
A time of use tariff (or TOU tariff) is a billing rate where the price of electricity changes across different time periods of the day and is usually driven by the demand for electricity. For instance, as the sun goes down, the amount of solar energy being generated drops, people arrive home from work and there is a sudden increase in the demand for electricity in the grid as people start cooking dinner, turn on airconditioners or heaters and lights.
So with TOU pricing, the amount you pay for electricity depends on the time of day that you use it. Generally electricity will be cheaper during the times when demand is low (off peak) and more expensive during the times when demand is high (peak period).
With a TOU electricity plan, it could be beneficial to consider how to shift as much of your electricity usage to the off peak periods to avoid paying the higher price.