Spool request explanation

A spool request is a document for which a print function has been selected. However, it has not yet been output to a printer or another device. The output data for the print document is stored partly formatted in a data store until an output request is created, that is, until it is sent to a particular output device.

The spool system uses a spool request to store the print data temporarily and to access it. The data is stored in a temporary format. You can also display the print document.

The system automatically assigns a 10-digit ID number to a spool request.

Also Refer

http://www.allinterview.com/showanswers/71855.html

SAP document itself can explain you very well

http://help.sap.com/saphelp_nw70ehp1/helpdata/en/24/ece847a9d511d1a9e40020af00b7e5/content.htm