What is game mod apk?

A mod (short for “modification”) is an alteration by players or fans of a video game that changes one or more aspects of a video game, such as how it looks or behaves. Mods may range from small changes and tweaks to complete overhauls, and can extend the replay value and interest of the game.

And then, Game mod apk is a game that has been mod for Android operating system, released as apk game