This is a static special that gives the floor of a sector a different light level from the remainder of the sector. The light comes from the sector in front of the trigger linedef. For example you can use this to have bright lava in a dark room.
Under the classic Doom format this special is numbered 213.
Under UDMF you can also use a sector's lightfloor and lightfloorabsolute properties, but this special is still useful if the source sector has a dynamic light effect, which will get transferred exactly to the targets.