The grid needs reserve power supply when there’s a failure in part of the system. For instance, some parts of the grid might witness a power failure due to generation or transmission issues.
The spinning reserve is the extra generating capacity available by increasing the power output of generators. For most generators, power output is increased by higher turbine torque. The non-spinning reserve is the extra generating capacity that is not currently connected to the grid but can be brought online after a short delay.
In isolated power systems, this equates to the power available from fast-start generators. In interconnected power systems, this includes the power available on short notice by importing / retracting power from connected systems.