Lux is the quantity of light or luminous flux incident on a surface to the area of that surface.

It is used when quantifying the amount of light falling onto a surface. Independent of the direction from which the luminous flux reaches the surface.

One lumen of luminous flux, unifromly incident on:
1 sqaure meter of area ==> 1 lux

P.S. It is also referred to as light level, illumination level or illumination value.