So what is in index.php?

Hello,

I'm guessing you are talking about WordPress by how you worded your post. In WordPress, index.php (when talking about templates) is an end-all catch-all file that is required for a template to work. This file is one of the requirements that WordPress looks for to make sure that a template isn't broken. Since you can have many differently-named template files for pages, posts, statics, etc, there is a chance that a page type may not have the template file that WordPress needs in order to compile that specific page. Remember the template hierarchy Zac explained? Index.php will have a basic generic default template structure that WordPress will use in case a template structure that could have been built for a certain page isn't there. Remember, all a template is is how the page is structured and looks so there could be many different types of templates for one type of page. This is just how WordPress gracefully catches missing template page types. So to answer you question (if it hasn't already), the index.php file is there as WordPress's last resort to render a template for a page, is required for a template to work, and renders a default simple generic template that can be used by every singe page created by WordPress. Hope this helps.

Cheers!

That was extremely helpful Thanks!