National Grid Reserve Service is a service offered by National Grid, the company that supplies energy to homes and businesses in the United Kingdom. The Reserve Service is like an energy 'emergency kit' that helps keep electricity flowing on the grid when there is a higher than usual demand for electricity.
When demand for electricity is too high, National Grid can ask companies to join the Reserve Service. The companies offer to reduce their electricity usage for a short period of time, by switching off certain machinery or other devices that need a lot of electricity. This helps keep the grid running and prevents a power cut. The companies are then paid for the electricity they don't use.