From Traderpedia

Definition: FIFO is an acronym for First In, First Out. This describes the time/priority principle of a queue system as used in the first-come, first-filled process of order matching at some exchanges.

An example of the FIFO system is used by Globex at the CME. Those who enter their order at the exchange first are filled handled first, whoever joins the order queu next waits until the first is filled.