Fifo stands for "First In, First Out". It means that if a certain number of people enters a place in a queue, then those people will exit the place in the same order.
For example, think of an elevator with doors on both sides. In 3 people enter from the front door and they exit from the back door, they will exit in the same order that they enter.
The opposite is the so-called FILO (First In, Last Out), in which people leave in the inverse order.
Imagine now an elevator with only one door. The first person who enters will be the farthest from the door, while the last person entering will be the closest. So, he will exit first, and the first person entered will exit last.
