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.
From Menu Style you can change menu items top , bottom, left and right space. You can also change DropDown Menu width.
From Slider Style you can change slider background overlay, text color, slider height etc.