A mod is a modified version of an application/game. Mods are created to make using the utility or pass levels easier. They are developed by third-party specialists or amateurs who are not related to the actual creators.
Modified products can be downloaded from the Internet. In most cases, they are freely available. And so pay for it, unlike the official versions, you will not need.
Modified applications are even available in official app stores. But before you install a utility from there, you need to ensure that it is suitable for the specific device by the required OS version.