Script Hook V is the library (created by Alexander Blade) that allows to use GTA V script native functions in custom *.asi plugins.
Note that it doesn't work in GTA Online, script hook disables custom scripts when player goes in multiplayer.
Copy ScriptHookV.dll to the game's main folder, i.e. where GTA5.exe is located. In order to load asi plugins you need to have asi loader installed, you can download it separately or use the latest version that comes with this distrib (dsound.dll). This distrib also includes a sample asi plugin - native trainer, if you need a trainer then copy NativeTrainer.asi too.
Report problems with download to firstname.lastname@example.org