A lumen is a standard reference for an amount of light. According to Wikipedia,
"if a light source emits one candela of luminous intensity into a solid angle of one steradian, the total luminous flux emitted into that solid angle is one lumen. Alternatively, an isotropic one-candela light source emits a total luminous flux of exactly 4? lumens."
Simply put, the lumen specification is a measure of the total amount of visible light emitted. For example, a standard 100 Watt incandescent bulb emits about 1300 lumens.