Blend mode

Also called: blending mode, screen, multiply

Every layer in an editor or compositor sits on top of the ones below it, and the blend mode is the arithmetic used where they overlap. Normal covers what is underneath. Screen brightens and treats black as transparent. Multiply darkens and treats white as transparent.

Those two do most of the work for creators, because they drop light leaks, flares and paper textures over footage with no masking at all.