The index.html page is the most common name used for the default page shown on a website if no other page is specified when a visitor requests the site. In other words, index.html is the name used for the homepage of the website.
If you need to quickly check that your domain is pointed to the correct server, as an example. Then you can upload the following code:
<!DOCTYPE html>
<html>
<head>
<title>Index Test Page</title>
</head>
<body>
This is a test page - If you can see this, then your index.html page is in the right folder.
</body>
</html>
Save this code as index.html and upload to the default web folder on the server, then visit the domain.
If you see the message that is contained above, then everything is working as intended.