A Fresnel lens is a type of flat lens which concentrically lines up many lenses with a triangular or other cross-sectional shape along a line. It is designed so that the light refracted by each triangle passes through the same point (focal point), and has the same function as a typical lens.
In a typical lens, the thickness of the material increases as you go from the periphery to the center. However, a Fresnel lens is unique in that because there is no change in thickness between the periphery and the center, it is thinner than a typical lens. Moreover, multiple lenses can be formed on one curved surface through resin formation.