Sorry if this is obvious but I bought the [GUI] Splatter item and can't understand what it does/what it's for?

It is an item that changes the appearance of the blood splatter on your Toribahs menu. When you open Toribash, there is a menu with single player, multiplayer, etc...It enables you to change what the blood splatter looks like.

For more explanations and images, look here http://forums.toribash.com/showthread.php?t=169313

It's an item to customize/make and upliad your own blood splatter in the menu....

It's an image in your toribash menu