O/D Button turned On or Turned Off

Hi I have recently purchased the used Hyundai Accent 2008 model and on the screen there is a light which saying o/d is turned off and when i press the o/d button on the gear handle then the light will go out...

when the light on the dashboard is on which says o/d is turned off.. What does it means it is On or Off..

I am confused now so can anyone help me please