These are services used in electric power systems to maintain the frequency of the grid within a specified range. FCAS helps to balance the supply and demand of electricity to make sure that the frequency remains stable. This is crucial for the proper functioning of electrical appliances and the overall stability of the power system. 

FCAS involves rapidly increasing or decreasing energy generation or usage to match changes in electricity demand, thereby keeping the frequency steady. This service is essential for maintaining a reliable and efficient electricity network.