Theme customizer allow you customize the site real time. You can see the changes at same time you make any change in customizer settings.
Most of the options in customizer are coming from the WordPress core, which allow you to customize site information, logo, menu, widgets, static front page etc.
From Style Settings in Customizer you will get 3 options. These are Header Style, Menu Style, Slider Style.
From Header Style you can change logo width, top & bottom space, header color, background color etc.