On-Off response time is the period required for a liquid crystal cell to go from an active state (black) to and inactive state (white) and back to an active state again.
It is time measured in milliseconds and generally a lower response time is better. A lower response time means faster transitions and, therefore, results in fewer visible image artifacts in the display with the transition of texts and graphics.
On-Off response time is a more important measure in the display of business content like documents, graphs and photos.