The document discusses the WordPress customizer, which is a visual editor and live-view for customizing WordPress themes. It allows flexibility for users and is simple for front-end users to use. The customizer contains settings, panels, sections, and controls. Settings store reusable customization options, panels group sections, sections group controls, and controls are the user interface elements like text boxes and color pickers. The document provides examples of adding these different customizer elements and recommends resources for learning more.