The camera
In the Camera view, the mouse and keyboard manipulate the Camera's position, orientation and field of view from the viewpoint of the Camera.
The yellow rectangle in the Camera view outlines that part of the viewed scene which will be rendered. This is defined by the dimensions of the rendered image and the field of view.
The screenshot also shows one of screenhack's features: an image or video (converted to a numbered sequence of images) can be placed in the background of the camera view.