you really need to explain what your trying to do here. are you trying to get the players hud or just a random widget? or maybe their camera view? user interface doesnt really tell us anything specifically.
casting basically is a way to identify the class of a object. its like asking is this thing (object in) like the class im casting to. so the object in will need to be a reference to the object your identifying. so if your trying to cast to a healthbar widget then you will need to have the object in be a widget. if your trying to cast to a hud subclass then the input should be a hud. in reality its all about inheritance and asking does the object inherit from this class.
also when you ask a question be very clear, provide details, and when its about a certain node include a picture or a link to the node in the docs. that way we know what your trying to do and which nodes your trying to use.
