On Look Ahead, or On Time, is the amount of time needed to open the boom valves and build pressure before entering an unapplied area. An example would be exiting your headland and going back into an unapplied area of your field. You increase the number to turn on sooner.
Off Look Ahead, or Off Time, is the amount of time needed to close the boom valves ahead of an already applied area. An example of this would be entering into your headland after spraying a pass in your field. You increase the number to turn off sooner.
A good setting to start with on your On Look Ahead is 1.5 seconds and the Off Look Ahead at .5 seconds.