What is the X in X-mas?
(FIFO, or "queue") A data structure or hardware buffer from which items are taken out in the same order they were put in. Also known as a "shelf" from the analogy with pushing items onto one end of a shelf so that they fall off the other. A FIFO is useful for buffering a stream of data between a sender and receiver which are not synchronised - i.e. not sending and receiving at exactly the same rate. Obviously if the rates differ by too much in one direction for too long then the FIFO will become either full (blocking the sender) or empty (blocking the receiver). A Unix pipe is a common example of a FIFO.
A FIFO might be (but isn't ever?) called a LILO - last-in last-out. The opposite of a FIFO is a LIFO (last-in first-out) or "stack".