FIFO stands for "First-In, First-Out". It refers to a data structure called a queue, which operates based on the FIFO principle.
In a queue, items are added to the back of the queue and removed from the front of the queue. The first item added to the queue is the first one to be removed, and the last item added is the last one to be removed. This is similar to a line of people waiting for a service, where the first person in line is the first one to be served, and the last person in line is the last one to be served.