This word is French and gets mispronounced a lot. FYI: it’s pronounced “freh-nell” — the “s” is silent.
This is a lens that gets cut into segments and smushed down to a thinner lens. These were famously used in lighthouses that needed super heavy, gargantuan lenses to shine big lights far out into foggy waters. By going to a Fresnel lens shape instead, lighthouse lenses could be made more thinly and of lighter weight.