U,V,W are surface coordinates similar to X,Y and Z that stay relative to their parent surface. They can be used for plotting additional surface topology, adding geometry and even (and most commonly) for applying 2D textures to your model’s surface.
A great video that I found from GuerrillaCG on the subject can be found here. I really recommend taking the extra few minutes to watch it. While you’re at it, I recommend watching their other videos as well. Very concise!
The 0,0,0 video can be seen here.