Imagine a situation you are driving a car at high speed. Suddenly you see a man crossing the road. Then you realize you want to stop the car. Then you press the brake paddle. There is short time period once you see the obstacle and start pressing brake. The action is that you see a hazard infront. The reaction is the pressing of brakes.
The short time period between seeing the hazard and pressing the brake is the reaction time. This is only one example. You can find dozens of these in life.(someone try to hit you and your reaction,A vehicle coming to hit you and you jump aside,a striker avoiding a defender in football etc.)
In highway engineering the above example is highly used to determine the safe distances for passing vehicles.In road marking this is speacilly considered. Because during the reaction time also vehicle travel some distance.
So once we see the obstacle we will stop the vehicle at a distance equal to breaking distance+reaction time distance
The time needed to response a situation, translated to distancerequired for this, depending on the speed of travel; breaking distance + reaction distance equals the stopping distance.