This is the intermediary device between a computer and a printer. In the old days, if you had no spooler, your computer would wait as the printer slowly printed a document. You would send your print data to a spooler to accept the data and save it temporarily to hard disk or memory while it deals with the slow printer for you. Nowadays, print-server is a more current term for describing this type of device. Most modern operating systems contain spooler processes that take care of printing in the background and you don't notice any delays anyway.