It is an API to access the functions of the WebUI built into BitTorrent/uTorrent for Windows. This is independent of the standard WebUI and requires nothing on the client-side besides the option being turned on.
The API is stable and largely complete. Missing functionality will be added over time and compatibility with existing applications will generally be preserved, so little to no work will be needed to keep your application working with future versions of BitTorrent.
Many other projects are available in the WebUI section of the forums. You can post your project there as well!