Websites provide a lot of web pages with different URLs but there are some defacto pages that exist most of the web sites. index.html is one of them. index.html is used as the main or entrance page for the whole web site or web application.
As its name suggests index.html consists of two words index and html. The index simply means shortlist for multiple items. In web terminology generally used to showcase the website pages, categories or parts on a single page. Html simply an extension for the index file which means the index file is an HTML file that contains HTML code or tags with some CSS and JavaScript code. index.html file generally resides in the root path of the web page and when the web site like http://www.poftut.com is requested by default index.html file is returned.
As stated previously index.html file contains mainly HTML code or tag with some CSS and JavaScript code. The client web browser will render and show the HTML, CSS and JavaScript code. For example, the following screenshot is the https://www.apachefriends.org/index.html. This is the main page of the apachefriends.org which contains content like text, header, video, menus, categories, links etc.