A LuminousFlux value represents the total amount of light emitted by a light source. You can think of it as roughly "photons per second", although it's a bit more complicated than that.

Luminous flux is stored in lumens. It's often used to describe the total output of a light bulb; for example, a 50 watt incandescent bulb and a 6 watt LED bulb might each have an output of 400 lumens.