The Layouts tab displays the 5 layouts available on documents (local types and built-in types for example) to let you customize the creation, edition, import, metadata and the view of your documents. Workflows and Page providers are also managed on this tab.
The UI tab gives you access to several elements to customize the UI section that you want, dashboard, menus, themes, default document view, integrate new custom pages, add new actions, add translations, etc.
The Resources tab represents the physical view of the package which is going to be deployed in the Nuxeo repository. It’s an alternative visualisation of all components created in the Layout and UI tab.