Answer: Reaction time & braking distance.
Explanation: When you're following behind a car, you're usually told to stay a car distance away from the rear of another vehicle, or at least two car distances away. Unfortunately, most people don't follow that anyways. In an emergency situation, much like someone slamming on brakes ahead of you, you are going to be expected to properly react and respond immediately - otherwise an accident many occur.
In order to do that, you need to make sure you keep your focus on the road. The reaction time is the amount of time it actually takes before you notice what is going on and work to stop it. The braking distance is solely the distance from the car and when you can brake, in my own terms. It's an easy topic to understand if you think of it from a positive, yet intrigued viewpoint. If you have more questions, ask!
