A lens is usually made from transparent glass or plastic. As light passes through a lens it undergoes two refractions. At the first surface, as the light enters the lens, it passes from air into, say, glass. Then the light travels through the glass. At the other side of the lens the light again refracts as it goes from glass to air. So, the light bends twice as it goes through the lens.
The surface of the lens is curved in such a manner so as to direct the light in a useful way. Positive lenses are designed to bring light rays together at a focus. Negative lenses spread light out.