Reaction time is the time it takes you to recognize there is a problem and for your body to physically react to the problem.
At first, you find that there is going to be collision. You realize that you need to brake to stop your vehicle.
After realization to make brake, you need to take your foot from accelerator to brake pedal. This is reaction time.
Then, you need to apply the brake by putting your foot on brake pedal or kicking it.