Appearance menu is most important menu in wordpress dashboard. How website or blog should appear? What content should be appear in website? Is totally depends upon Appearance tab You can apply themes, you can create menus by using Appearance menu. Appearance menu contain following sub menus
Themes: WordPress provides collections of themes. There are more than 2000 themes with different categories.
Customize: All themes can be customize here.
Widget: You can put different kind of widget in sidebar.
Menus: You can put menu bar using this option.
Theme Options: You can change settings of theme here.
Edit CSS: CSS can be customize using this option.
Editor: If you know coding you can edit code here. Don’t use this option if you don’t know coding. If you still want to use this option I strictly recommend to take backup of your website before changing code.