This is just a visual test file, and an ugly one at that! For unit tests, visit the Respond.js unit test suite
The media queries in the included CSS file simply change the body's background color depending on the browser width. If you see any colors aside from black, then the media queries are working in your browser. You can resize your browser window to see it change on the fly.
For a realistic use case for media queries: read up on Responsive Web Design
Media-attributes are working too! This should be visible above 37.5em.