The "mark" is a small X-shaped target on the display which is treated like a second origin (the normal origin is the upper let corner of the board). The GUI will display a second set of coordinates for this mark, which tells you how far you are from it.

If no argument is given, the mark is toggled - disabled if it was enabled, or enabled at the current cursor position of disabled. If the <arg>Center</arg> argument is given, the mark is moved to the current cursor location.