As a rule the file called index is the default start page. This means that this page will be shown first when you access your domain, directly (www.example.com). Furthermore index.php will always be shown before index.html, meaning that for example your Wordpress (php) site is shown and not your Website Builder (html) site.
With .htaccess you can change which file will be shown first and in what preferred order. You can for example use this:
DirectoryIndex home.html index.html index.htm index.php
Adding this line of code will by default show home.html when you go to your website. If that file is not available the server will try to load in respective order: index.html index.htm index.php, provided these files are on your web space in the same directory.