On Steam, it's 'Steam\steamapps\common\Grand Theft Auto V'. Take dinput8.dll and ScriptHookV.dll (and the trainer, if you want it) and put them in your GTA 5 game directory, wherever GTA5.exe is located.
It allows for all sorts of cheats in single-player, like changing your skin, changing the time of day, teleportation, invincibility, and more.
