Light refracts, or bends, at the interface of two materials (air and lens). When light hits one outside surface on a prism, its direction is changed. When the light passes through the prism and hits another outside surface, its direction is changed again. By carefully crafting prisms to the right specifications, the Fresnel lens takes advantage of these properties so that the glow of a light source can be focused into a solid, more intense beam of light.
(animation above: how the prisms of a Fresnel lens refract light )
(photo above: a close look at the prisms of the 1st order Fresnel lens in Point Arena Lighthouse, California)