Lumen (lm) is the internationally standardized unit of measurement for the luminous flux of a light source and thus allows conclusions to be drawn about the brightness of the illuminant. Colloquially, one also speaks of the light output of a lamp.
The luminous flux indicates the amount of light generated per unit of time; the unit of time is quasi = 0 due to the speed of light of 300,000 km/second, since the speed of light forms our perceptual limit.
Lumen is not to be confused with the unit for illuminance lux, which indicates how much light per unit of time is incident on a unit of area, i.e. the amount of brightness on/at an illuminated surface.