These grid lines are often also referred to as
paper, although you are free to define rectangular instead of
square raster grids if you wish. You can also set the seperation of
the grid lines and are not limited to the usual millimeter/centimeter
markings or their Imperial equivalents.
Although grid lines help to create a layout quickly, they later act more like a brake and slow down screen redraws. Therefore you are free to switch them on and off any time while working on a document.