A default page is the webpage which the server loads when no page is specified.
For example, if you try to open a website with the URL http://cern.ch/mywebsite/myfile.htm, it will load "myfile.htm" from the root folder of the website.
But if you try to open a website writing only http://cern.ch/mywebsite/, the server will search the root folder of the website, and if it finds a document with any of these names, it will open it. If there is more than one, it will open the first one in the order of preference.
This is the list of accepted names for default pages (in order of preference, from top to bottom):
If there are no files with these names, and "Directory browsing" is not enabled in the Web site configuration, the server will produce an error with the code 403.14.