Spooling stands for Simultaneous Peripheral Operations O-Line. It is referred to as the act of putting data of various I/O jobs in a buffer.
A buffer here refers to a special area in memory or hard disk that is accessible to an I/O device. It is used for mediation between a computer application and a slow peripheral. It is very useful and important because devices access or acquire data at different rates. This operation also uses disk as a very large buffer and is capable of overlapping I/O operations for one task with processor operations for another task.