The time from initial movement of the pitcher’s hands from the set position until impact of the ball with the catcher’s mitt (or impact with a net if throwing into a net).
This time is important because the less time it takes for a pitcher to get the ball to the catcher, the less time a runner has to steal a base. Time to Plate is made up of delivery time and ball flight time (the time it takes the ball to get from pitcher’s hand at release to the catcher). A pitcher can focus on controlling his delivery time to reduce the total time to plate.
Real Life Example:
On a field with 90-foot base paths, it takes a runner just over 3 seconds to steal second base. If a pitcher can keep his time to plate to 1.3 seconds instead of 1.5, then the catcher has more time to throw out the runner.