for people with normal vision, light rays enter the cornea in the front of the eye and are focused into a single point on the retina in the back of the eye. Once it hits the retina, the light is converted into signals, which go to the brain to be processed into images.
sometimes the cornea doesn't focus light precisely on the retina because of a refractive error. The contact lens refracts, or bends light, so that it focuses correctly on the retina. its shape is based on the type of vision problem that needs to be corrected. how much the lens bends light, or its strength, is expressed in diopters. the higher the diopter, the stronger the lens.
hope this helps answer your question :)