Answer:

The curve called a parabola has the property that when rays of light (or heat) parallel to the axis of the parabola are reflected by the curve, they all pass through a single point. This point is called the focus of the parabola.

The principle is used the other way round in a car headlight; the cross-section of a headlight is a parabola, and the bulb is at the focus. The reflected rays of light then form a parallel beam.