The Index Page is the first page that loads when someone goes to your website. It is also known as the Home Page.
By default, it has a URL or site address of http://___(YOUR DOMAIN)___/index.html
The Index Page usually has a different or "page-specific" layout than the rest of your site. Any changes made to the layout of the index page will not affect any other pages in the site.
To make site-wide changes, you will have to edit the Master Template. Unlike the Index Page, when the Master Template is edited, the changes will reflect in all the pages associated with it.