To spool (which stands for "simultaneous peripheral operations online") a computer document or task list (or "job") is to read it in and store it, usually on a hard disk or larger storage medium so that it can be printed or otherwise processed at a more convenient time (for example, when a printer is finished printing its current document). One can envision spooling as reeling a document or task list onto a spool of thread so that it can be unreeled at a more convenient time.
Printing terms: Glossary
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The idea of spooling originated in early computer days when input was read in on punched cards for immediate printing (or processing and then immediately printing of the results). Since the computer operates at a much faster rate than input/output devices such as printers, it was more effective to store the read-in lines on a magnetic disk until they could be conveniently printed when the printer was free and the computer was less busy working on other tasks. Actually, a printer has a buffer but frequently the buffer isn't large enough to hold the entire document, requiring multiple I/O operations with the printer.
The spooling of documents for printing and batch job requests still goes on in mainframe computers where many users share a pool of resources. On personal computers, your print jobs (for example, a Web page you want to print) are spooled to an output file on hard disk if your printer is already printing another file.