To use Script, you first need to load GUIKit using Needs["GUIKit`"].
Script is used to associate scripting code either as general code to be executed when the widget definition is read in or as code bound to a specific event, to be executed each time the event occurs within a BindEvent expression.
By default, the language of a Script is Mathematica code unless an alternate language is specified. You can use Language -> "xml" to specify that the content of the Script is a string of GUIKitXML.
Script is HoldAllComplete so that evaluation does not occur when processing Script blocks.