The Customizer is a feature that lets you live preview your site and see changes on the theme as they are made. Depending on your theme, you will have more or less features located in the sidebar for you to customize. At the top, you will see a brief description of what the customizer does.
Again, depending on your theme set, you will have the option to change your site title and tagline, use a color selector or color values to change your title and background color, add and crop header images, set a background image and choose your navigation menus.
Clicking widgets will show you all the available places you can place your widgets. Add new and reorder widgets and see the changes immediately. Choose what you want to display on your front page and choose how featured content is displayed.
When you are done customizing, don’t forget to click Save and Publish.