To post it as an answer, not only as comment:
This is explained in the documentation, see doc groot
You can search in the documentation also:
A short summary: The root object is the parent object of all figures. Some of its properties belong to the Matlab session, as e.g. the CurrentFigure. Some concern the current computer setup as the ScreenSize or the PointerLocation.
In older Matlab versions the graphics handle of the root object was 0. See: