The various Appearance menus allow you to change how your site looks and behaves. If your site has been setup for you by a third party then it’s more than likely that you won’t need to change any of the settings in here.
Depending on the theme that your site is running, you may find that you don’t have all the menu options listed here, or you may have a couple of different ones. The reason for this is that a couple of the options under the Appearance section are theme based. For example, the default Twenty Seventeen theme allows you to customize options for how your theme displays. Not all themes will have these option pages, or if they do, they may be under another menu entirely.