a focusing aid, once very frequent in manual focus cameras. It consists of an area of the focusing screen that has been covered with tiny split-image wedges: the effect is that, while the image is unfocused the microprisms look very "raspy" and ragged - but when the image is in focus the microprisms "snap" and the image turns clear.
Usually present in the center of the focusing screen in the form of a circle, occasionally reduced to a ring around a much larger split-image telemeter
They turn black in low light conditions, or when used with lenses that have an insufficient aperture - which, incidentally, is when you would need them more.