Newton's Third Law of Motion
Newton's third law of motion states that for every action, there is an equal but opposite reaction. For example if you wreck into a concrete column you will go forward with force but then your seat belt holds you back with the same amount of force when you fly forward. When you are going forward and you are in midair, you don't keep going because the seat belt acts as the opposite reaction. It stops you from slamming into the window.
If you still don't understand, we have another example. If you hit a car from the front, the seat belt will stop you from going forward and you won't fly out on your face. The cars have different masses so if a small car hits it wants to go backwards right when it hits the small car because the bigger car has a larger mass and it is applying a force on the smaller car. For the bigger car it is a different story. It wants it stay right where it is because the small car has a smaller amount of mass. This is called a reaction force.
If you still don't understand, we have another example. If you hit a car from the front, the seat belt will stop you from going forward and you won't fly out on your face. The cars have different masses so if a small car hits it wants to go backwards right when it hits the small car because the bigger car has a larger mass and it is applying a force on the smaller car. For the bigger car it is a different story. It wants it stay right where it is because the small car has a smaller amount of mass. This is called a reaction force.