Your "index.html" or "index.htm" file is the main file of your website. This is the defualt page that people will see when they visit http://www.yoursite.com
<br>
<br>You can change your index html file to be any page you want, but this will require some editing of the configuration files on your webserver. 99% of all websites use index or a slight variation for their main page. If you are unsure what your index page should be you can contact your webhost. They will be able to tell you what to use.
<br>
<br>Your index.html file should tell the use exactly what you site is about and be inviting enough that the visitor continues to browse your content.