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 download one of the pre-built releases from the html5media download page.
To run the build script you'll need a copy of python 3 installed on your system. Assuming you have this, building html5media is simply a case of opening a terminal and running the build script like so:
$ bin/build.py
The built html5media code will all be placed in a brand-new build/ directory
in the root of the project.
Happy coding!