What does the LPD printer service mean?

Line Printer Daemon (LPD)
Line Printer daemon, a UNIX daemon (background service) for spooling printing work.
The Line Printer daemon is a background program installed on a UNIX print server. Its function is to wait for customers to use the Line Printer remote (LPR) protocol to print work.
When LPD receives a print task, it first registers the print task in the print queue, which is a subdirectory of the file, where many of the print work waits for LPD to process.
When the print device is idle, LPD takes the print job out of the print queue and passes it to the printer for printing.

What does the LPD printer service mean?