A lumen is a standard unit of light as it is perceived by the human eye. Essentially, it will tell you how bright something is, unlike a watt, which is actually a measurement of energy consumption or output.
The lighting industry has standardized ways that it measures the total emission of light from a product and include it on most packaging.
How bright is a light bulb?
Because the Lumen output from different manufacturers will vary. Here's a quick and easy guide to approximate total Lumen output.