FIFO stands for First-in First-out. Meaning it is a queue and the first data that enters it is the first one to leave. Think of a concrete sewer pipe that is placed on a playground and if a group of people enter one end and their direction of movement does not change the first person in will be the first person out. That's a FIFO. Had the pipe one end blocked the movement would have to reverse and you have a LIFO (last-in first-out).
In Unix a fifo is a named pipe. See Fifo.