The Customizer is a feature that lets you preview the themes on your site and see changes before they are made live.  The sidebar contains all of your Customizer options.  The options available to you will depend on what theme you are previewing.  At the top is a brief description of what the Customizer does.

Clicking “Change” will bring up your available themes which you can select to live preview.  Under Site Identity, you can set the Site Title and Tagline. Site Icon, which is an image that will represent your site when it’s seen by browsers and a Site Logo.  Again, depending on your theme, you may or may not have these options.

You can select your colors, load a header and background image or video and select which menu you wish to use for navigation.  The widgets tab will let you see all of your widgets areas and and see the changes in real time when adding or rearranging widgets.

You can choose whether to display your latest post or a static page as the homepage of your site. And under additional CSS you can make style changes to your theme by adding custom CSS.

When you are done editing your pages, don’t forget to click ”Save” and “Publish.”