What is braking distance?

Other answer:

Conneh V:
Distance measured from the time you step on the brakes till you stop.
Scott H:
The distance from when you first apply the brakes to the point where you are at a complete stop.
The distance it takes to come to a stop by using brakes.
Unknown Name:
what part of braking distance don't you understand?
Look at your driver's education manual and learn what it is.