This is a randomly-generated URL for the preview for your pull request, and a new one is generated every time you make a change. This is not just a single lesson, but rather a self-contained instance of the full NRE Labs platform. This means that your preview URL will take you to what looks like the main NRE Labs site, but in fact this site is updated with the changes you've made. So, to see your changes, go through the lesson catalog and find your lesson, and launch it, just like you would on the main NRE Labs site.