These nodes are for capturing input from a mouse or touchpad.
The composition must have at least one window for these nodes to output any mouse information.
If you leave the Window input port of a mouse node unconnected, then it will output mouse positions in screen coordinates. With a single screen, the top left corner is at position (0,0), and the bottom right corner matches the resolution (width and height) of the screen in points. For example, a 1440x900 display would have bottom right position (1440,900).
If you connect the Showed Window output port of a graphics window node (such as Render Scene to Window) to the Window input port of a mouse node, then the mouse node will output mouse positions in Vuo Coordinates, relative to the graphics area of the window.