On desktop, the customizer has a preview of the website on the right and the customizer settings panel on the left. You can preview the site in different screen sizes by clicking the device icons in the bottom right of the settings panel. On mobile, the customizer settings panel is full-screen, and you can click the “Preview”-button in the top to show a preview of the site.
As default, the following settings are included in the WordPress customizer:
More features can be added to the Customizer by themes and plugins. The big benefit of the customizer is that you can preview your changes, which makes it more pedagogical than editing your menus through Appearance → Menus or widgets through Appearance → Widgets. The customizer was introduced to WordPress in version 3.4, released in June 2012.