So the title has two meanings: the literal and figurative.

 

The word light in the literal meaning references to the wavelengths produced by radios. They wavelengths are the super tiny particles that get transmitted from one radio to another. The radio played a big role in World War II and it symbolizes hope in this novel. Although the Germans and French were against each other during the war, they were all interconnected through the music, literature, and news that played on the radio. Figuratively speaking, the light we cannot see references to our stubbornness to not see the good in people because we were taught not to. Werner refused to listen to Jutta talk about how the French were the victims. He blindly believed what Nazi school had taught him: that Germany was trying to regain its power from World War I and that everyone else was in their way.  The title also references to Marie being blind (literally). Although she can't see a thing, she is capable of seeing the light in people and in the world around her.