What does spool mean for printing?

Mark wrote a comment for me > I don't know offhand how to make cups not spool, that is, how to make the lpr command only exit after the printer driver has run. What does "spool" for printing mean? Google says it is a verb meaning "send (data that is intended for printing or processing on a peripheral device) to an intermediate store." What is the intermediate store that printing spool represents, for example, when printing by lpr command Mark seems to relate the meaning of spool with blocking. But I can't figure that out by looking at the definition given by Google. Thanks.