Lux is a measure for illuminance, i.e. the amount of light passing through a surface or reaching the eye. It therefore determines the effect that light has on you.
Obtained from a light spectrum
The lux value can be obtained from a light spectrum by weighting it with the ocular receptor spectral response: each type of ocular receptor is predominantly sensitive to a certain color of light. Depending on the receptor in question, one can for instance distinguish photopic, scotopic, melanopic lux.
Lux most often implies photopic lux, characteristic of color vision under daytime conditions. A candle at 1 meter distance gives 1 photopic lux of light. Typical room illumination is 300-500 lux, whereas outdoor light varies from 1500 lux on a cloudy day to 100000 lux on a sunny day. Photopic lux describes an average response of the three color vision receptors (cones), and it has been used for years to describe light therapy devices.