This article is from the Beer and Other Brewing FAQ, by Kurt Swanson email@example.com with numerous contributions by others.
For brewers, the Lovibond degree is a unit used to measure the color
of malted barley and beer. Darker grains have a higher Lovibond measure,
and contribute more color to brewed beer. Darker crystal malts (such
as 60L, 80L, 120L, etc.) will provide more sweet flavor and more color
than similar amounts of lighter (20L, 40L) crystal malt. Dave Miller's
book (see Bibliography) provides a formula for very roughly predicting
the color of finished beer in degrees L based on the grain that goes
into making the beer.