On the Customization tab, you can edit & manage the storefront theme, design, navigation, add new pages and fully customize the look & feel of your storefront.
Customize - Open the theme editor to make basic changes to your Home page, Catalog Page, Content Page, and Log-in Page.
Backups - Create a theme backup prior to making changes so you can revert back to an old theme if needed. We recommend creating backups regularly just in case!
Navigation - Control & modify your header and footer navigation here.
Advanced - Includes the Code Editor (You can modify the template using HTML. Templates are built with Liquid, which is a simple and powerful language), Code Snippets (You can add your own custom CSS code to change site appearance), and Assets (Upload images/other assets and link to them via HTML).
NOTE: If you're looking to further customize your look & feel and need a developer to help you, check out this post on hiring a developer.