The interface that you have designed contains a complete description of the layouts and menus that are available, but does not yet specify what any of the various elements do. To do this, you need to specify callbacks in the interface definition. As you might expect, this is done by setting attribute values for the appropriate elements in the interface.
In this example, the callbacks that you supply are calls to other functions, the definitions for which are assumed to be available in a separate source code file, and are discussed in Defining the callbacks. Note that you do not have to take this approach; you can just as easily specify callback functions within the interface definition itself, using lambda notation. It is up to you whether you do this within the Interface Builder, or by loading the code in the editor. If you choose the former, note that it may be easier to use the code view, rather than typing lambda functions into the Attributes dialog.