His first law talks about "inertia" which is the tendency of a body to keep doing what it's already doing. If it's moving, it wants to keep moving. If it's at rest, it wants to stay at rest. The moving part is important in car crashes. If the car and you are moving, and the car suddenly stops (as in a crash), you will keep moving at the speed you and the car were moving at until you are acted on by a force (the windshield or the dash). No one wants this to happen.
Newton's second law relates force, mass, and acceleration. Force is required to give a mass acceleration. The bigger the force, the greater the acceleration, if the mass remains the same. In Physics talk, acceleration is a change in velocity. It can be either speeding up or slowing down. Another important concept that Newton gave us is momentum. Momentum involves the mass of an object and its velocity. A heavier object traveling at the same speed as a lighter one has more momentum. An object can have a large momentum by having a large mass, a large velocity, or both.
So, if you apply a force to an object of constant mass, it will experience an acceleration. If you apply the brakes to your car to slow it down, it will experience a change in momentum. This takes some time in normal cases, so time plays a role as well. The product of force and time is call impulse. The faster a force is applied, the more damage is done. If we slow down the time over which the force is applied, we will lessen the damage done by the force. Seat belts, air bags, and the crumple zone of your car are each designed to lengthen the time that the force is applied and thereby lesson the damage done by the force.
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