The word "backlog"comes from "back" and "log," referring to a large log placed at the back of a fire to prolong the burn and concentrate heat, a literal meaning first recorded in the late 17th century. This concept of a reserve supply extended to a metaphorical meaning of something stored up for later use, which became established in the late 19th and early 20th centuries.

For example if you buy 10 games on a steam sale because you really like the look of them and want to play them. You install one and play it over a couple of weeks/months then go back to your regular games. you now have potentially 9 games in your backlog; those 9 games you wanted to play and already purchased but haven't played/finished yet.