Guard rails are intended to protect people by preventing cars from leaving the sides of the road near cliffs, rivers, curves, and bridges. Most of the time they do this well. Unfortunately, some older guard rails actually pose a threat to lives. If the guard rail does not end by being curved to the ground, or in a breakaway design, the rail may be capable of impailing a moving car and its occupants. The ends of these older rails are usually sharp and difficult to see from the end. A rapidly moving car upon running into one is met with a very large force and high pressure as that force is exerted on a very small edge of the rail. The effect is known as "guard rail spearing".
Crash footage from the National Transportation Safety Board