Luminous flux is the quantity of light energy per unit time arriving, leaving, or going through a surface.
If you think of light as particles (photons) moving through space, then the luminous flux of a light source measures the number of particles leaving the light during a time interval of 1 second.
The lumen (lm) is the unit of luminous flux in both the International System (SI) of units and in the American System (AS) of units.
In 3ds Max, luminous flux is one way to specify the intensity of a photometric light. See Intensity/Color/Attenuation Rollout (Photometric Lights).