The Fresnel effect is a natural phenomenon that describes the way we perceive light and reflections as they bounce off surfaces and into our eyes. Sometimes called the “storefront effect,” in that viewing storefront windows head on will show few reflections, whereas glancing views will how much stronger reflections. The Fresnel shader addresses this effect directly, and offers control to tune the effect to your preference. Prior to the introduction of fresnel shaders in computer graphics, reflections look very stylistic; interesting, but not real.
Fresnel shaders are found in many different material channels, not just reflections. In some cases, other shaders may directly incorporate the Fresnel effect, so that you won’t have to add the shader in yourself.