A fresnel lens is an accessory for a groundglass focusing screen. It helps "even-out" the brightness of the groundglass image and make the image appear brighter. The effect is more noticeable with wide angle lenses. Fresnel lenses are essential when using reflex housings.
Toyo offers their 4x5 fresnel in two versions; one for cameras that never had a fresnel (TOF45K) which includes the sandwiching clips that hold the fresnel and groundglass together, and the fresnel lens ONLY (TOF45) for replacement of existing fresnels.
Note: A fresnel lens is standard equipment on all current 4x5 Toyo cameras except the the 45C and the 45CX.