Time of use (TOU) pricing applies different charges to electricity usage (in kWh) at different times of the day (or week). Days are commonly split into peak and off-peak (and sometimes shoulder, which is the time between peak and off-peak) periods.
Peak periods, where the electricity is most expensive, are intended to correspond to the times the network faces high demand, but in practice are wide periods that cover much of the day.
Off-peak periods, where the electricity is least expensive, are priced to correspond to the times the network faces lower demand.
Shoulder periods are in the middle - these are priced higher than off-peak and lower than peak, again to influence demand of electricity from the grid.