How to use Graphic Raycaster with WorldSpace UI?

I'm trying to figure out how Graphic.Raycaster works, but documentation doesn't help. I want to use it to cast raycast from some position an certain angle and hit UI. The other thing is that I don't know how to make it interact with UI(drag, click etc.). I know that's broad subject, but I just can't find any good explanation of what it actually is and how to use it, so I would be grateful for any explanation.