These nodes are for working with screens (monitors or displays).
The dimensions of a screen are measured in points. For non-Retina displays, points are equivalent to pixels. For Retina displays, each point is 2 pixels.
Positions on a screen are given in screen coordinates. If you have a single screen, its top left corner is at the point (0,0), and its bottom right corner matches the resolution (width and height) of the screen in points. For example, 1440x900 display would have bottom right coordinate (1440,900).
On macOS, you can look up information about the connected screens in System Preferences > Displays.