There are other individual pages in your PrestaShop install that would certainly benefit from friendly URLs: CMS pages, user account pages, pages with automatically generated content... The "SEO & URLs" page presents you with a list of these pages, and enables you to edit their friendly URLs as well as their meta tags (title, description, keyword).
Friendly URLs only work with a server setup that supports URL rewriting. Make sure to check that yours does (ask your hosting provider!), as it can make your shop completely unavailable to customers if you enable friendly URLs and the server does not support it!
Therefore, in order to change the homepage meta tags, you simply need to open the "SEO & URLs" page, click on the "index" page, and there you can freely edit some important SEO information.
A few tips:
Note that if your store has already been indexed by Google or any other search engine, it might take a while for your changes to appear in the search results. Please be patient.
At the bottom of the "SEO & URLs" page are three sections with additional options: Set up URLs, Set shop URL and Robots file generation. See below for an explanation of these.
Adding a New Friendly URL