In the old and moldy days of the internet, you could pop out to a site and see a list of files. Kinda dull. If you wanted to jazz up your list of files, you could make your own list in index.html. Over the years, people modified index.html to show content other than the list of files.
So, now, young grasshopper, you know why the file is called "index.html"!
Since we're now doing jsp stuff, we could have called the file index.jsp. But we would have to go through the steps of telling orion that the default file is "index.jsp" instead of "index.html".