If you can see this file, then you've created a clone of the html5media GitHub repository or have downloaded one of the source tarballs. This is perfect if you want to help develop html5media.
If you simply want to use html5media in your site, it's recommended that you simply link to the html5media CDN script, as explained on the html5media project site.
To run the build script you'll need a copy of nodejs and gulp installed on your system. Assuming you have this, building html5media is simply a case of opening a terminal and running the build script in the root of the project directory like so:
$ gulp build
The built html5media code will all be placed in a brand-new build/ directory
in the root of the project.
Happy coding!