It's like a 3D environment and basically sets the viewport and a 3D simulator to be where the camera would be. And you can actually put multiple viewports in there where you have multiple views on the same scene or you can render all the sensor information the robot has. Or if he has a laser in the base, you can render the laser scan itself and you see how good does it fit with the walls where they actually are. It's very good for you to get in the sense of why the robot is doing something because the more complex it gets, the more difficult it is for the developer to follow what's actually going on in the system.