A spool is a temporary storage area within the computer's RAM that contains input or output data. When a job or process is initiated on a computer, but cannot be run immediately, it is often placed in a spool. This process is called spooling. The spool holds the data until the appropriate device is ready to use it.
The most common type of spool is a print spool, which stores print jobs that are sent to a printer. Since the printer may need to warm up before printing a document, the document may be held in the spool during the printer's warm up process. If there are multiple documents that have been sent to the printer, the print spool may contain a queue of jobs.
You can think of a print spool as a spool of yarn that is connected from the computer to the printer. Of course, the print spool contains computer data, rather than yarn. As the the data is transmitted to the printer, it is removed from the computer, just like unraveling a spool. Once the spool is empty, no more data is transmitted to the printer.