What is the different between readme.html and index.html in github.io?

I have created github.io portfolio.

May I know what is the difference between readme.html and index.html files?

If I want to update my profile do I have to change the syntax on both the HTML files?

What my experience is — I only need index.html file to update my profile when it links to other pages (HTML files) like. If so what is the purpose of keeping a readme.html file?