This is the first of submodule repositories that host source code for a particular step of the running example of the article Get a Handle on Handly.
Each subsequent step takes the source code of the preceding step and adds some more functionality. In that way, you can always checkout example plug-in projects from the repository corresponding to the previous step and use them as a basis for further development in accordance with the tutorial's instructions for the current step.
See Step Zero of the article for information on setting up the development environment for the example.