What does this do?
Sorry to dissapoint, but at the moment this is a placeholder for future changes. Currently the most up-to-date C syntax highlighting can be found in the Better C++ Extension, however it is planned (eventually) to seperate that repository and move the C syntax highlighting into this repository for better organization.
In the future this extension should be the source of C syntax highlighting for VS Code. Installing it will get you the latest "bleeding-edge" features, and reporting bugs here will speed up the development of highlighting. Once a specific highlighting feature is well tested (which can take a while) it is no longer "bleeding edge" and will be merged into VS Code itself.