The automatic bearoff option enables GridGammon to automatically move for you when you are bearing off your checkers.
Auto bearoff will only function when two conditions are met:
you are in a no contact position (all your opponent's checkers are past all your checkers), and
your roll would allow you to bear off the maximum number of checkers (four if you rolled doubles, two otherwise).
You can enable or disable auto bearoff by opening the Board window, and selecting View > Window Settings... from the menu. Check or uncheck the Auto bear off maximum checkers checkbox.
Many players just set this option and leave it, but if you prefer to enable or disable auto bearoff more frequently during game play, you can add a toolbar button to the Board window. Select View > Customize Toolbar... from the menu, and drag the Auto Bearoff icon to your toolbar.