Despite its deceptively bland name, ScriptHook could be a big deal for GTA 5 modding. First, though, let's quickly cover the important stuff. It lets you be a deer! And a fish! And a chimp! And change lots of other game values, such as tweaking the world state, physics and weapon behaviour, or just teleporting around the world.
Here are a couple of videos that show the trainer in action.
ScriptHook doesn't work in multiplayer, which should negate any potential ban Rockstar might decide to hand out to people messing with GTA Online's innards. Unfortunately, it's hard to be sure. Rockstar hasn't really clarified their position on mods in GTA 5—we've previously asked them what they consider to be acceptable, but didn't receive a reply.
More than a way to be a cat for a bit, ScriptHook is a big step toward opening up GTA 5 modding. Essentially, it allows modders to add scripts; letting them create new things without overwriting existing bits of GTA 5.
At the most basic level, it means we're getting closer to replicating the historical high-point of modding: