Spinning Reserve in a power system refers to the reserve capacity of a source which is already connected to the grid. For instance, a 150 MW plant running at 80 MW may be considered part of the spinning reserve.
The spinning reserve in this case is 70 MW which is the reserve capacity. This can be brought into the system if the demand increases. The spinning reserve must be able to supply the required power within 10 seconds.
Non spinning Reserve in a power system refers to the power sources which are presently not in line by can brought in to the system if the demand rises. The non spinning reserve should be able to supply the load within 10 minutes