Now that you have a working project you should use the Yesod book to learn more. You can also use this scaffolded site to explore some basic concepts.
This page was generated by the getHomeR handler in Handler/Home.hs.
The getHomeR handler is set to generate your site's home screen in Routes file config/routes
The HTML you are seeing now is actually composed by a number of widgets, most of them are brought together by the defaultLayout function which is defined in the Foundation.hs module, and used by getHomeR. All the files for templates and wigdets are in templates.
This is an example trivial Form. Read the Forms chapter on the yesod book to learn more about them.
And last but not least, Testing. In tests/main.hs you will find a test suite that performs tests on this page. You can run your tests by doing: