Stands for "First In, First Out." FIFO is a method of processing and retrieving data. In a FIFO system, the first items entered are the first ones to be removed. In other words, the items are removed in the same order they are entered.
To use a real world analogy, imagine a vending machine where the items are loaded from the back. When someone selects a Milky Way bar from row E5, the machine churns out the candy bar closest to the front. The next Milky Way in line then moves to the front. Therefore, using the FIFO method, the candy bars are dispensed in the order they were placed in the machine.
Computers often implement the FIFO system when extracting data from an array or buffer. If the first data entered into the buffer must be extracted first, the FIFO method is used. The opposite of FIFO is LIFO, in which the last data entered is the first to be removed.