What is the index.html?

In the Management Console under section "Storefront & Websites" there is one page displayed named "index.html".

The index.html is the homepage of your storefront.

There are two different types of pages to set up and to be displayed in your storefront. You can chose from 'content pages' (including the homepage / index.html) and 'destination pages'. With regard to its origin the index.html is a content page as well.