Lux is an unit of measurement of the light intensity. It is also called illuminance which is the amount of luminous flux per unit area. Wikipedia may be of some help for you if you need further explanation.
However, just to give you an idea of what is like, 1 lux is equal to the illumination of a surface one metre away from a single candle. Some further examples is shown in Mr D Tail's post; the light levels are 200 lux in a bright living room, instead outdoor is about 100,000 lux in a direct sunlight.