This is a sample project to build a Canvas for Pebble plugin. The documentation on how to build these plugins lives here - there is a sample plugin included on that page (CanvasNowPlaying), but this is a much simpler variant. All it does is display a message on your watch's screen to say whether it's plugged into external power or not.
If I was going to tell you what to do, I'd tell you to download both of these and maybe look at this one first. This one contains only the raw basics - the other one actually demos use of all the features in Canvas plugins.