Blend mode
The rule for mixing a top layer with the one beneath it, such as multiply, screen or overlay.
See it in Noise & texture. Drop a picture on the noise, then switch the overlay between Multiply, Screen and Overlay.
In plain words
When one picture sits on top of another, something must decide the colour you see. Normal mode shows the top picture. Multiply darkens, as if two slides were stacked in a projector. Screen lightens, as if two projectors shone on the same wall. Overlay deepens darks and brightens lights, which adds punch.