The standard Temple Run 2 operates on a Freemium model. You earn coins and gems slowly, or you buy them with real money. A "Mod APK" is a repackaged version of the game's installation file (Android Package Kit) that has been reverse-engineered to bypass the game's normal rules.
In the vanilla game, you must achieve specific scores or pay gems to "travel" to new maps. This mod unlocks the entire globe: --- Temple Run 2 Mod Apk All Maps Characters Unlocked