Explanation of :GetMouse and :AddItem please?

First of all, the colon tells you that this is a method call of some kind.

GetMouse is used in a LocalScript to get a PlayerMouse object, which is similar to the Mouse object used in Tools and HopperBins, but requires neither.

AddItem is used to, well, add an item to Debris, where it will be removed from memory after some kind of delay. (This is useful for things that may or may not exist when you want to Destroy them, such as particle effects.)