A light bomber is a military bomber aircraft which, when compared to other bombers, is relatively small and fast; such aircraft will probably not carry more than one ton of ordnance.
Prior to WWII, engine power was so scarce that there were several types of bombers, light, medium and heavy, all tuned to a particular performance niche. As fighters grew to be able to carry the same sorts of loads at even greater speeds, light bombers were replaced and the term fell from use.
The light bomber accomplished missions similar to that tasked today to attack aircraft and fighter-bombers.