Shannon’s answer to “What happened to the Sea of Flames stone at the end of the book? Was it still in the sea?” > Likes and Comments

This is from his website, in the FAQ section about the book:

What does the title mean?

It’s a reference first and foremost to all the light we literally cannot see: that is, the wavelengths of the electromagnetic spectrum that are beyond the ability of human eyes to detect (radio waves, of course, being the most relevant). It’s also a metaphorical suggestion that there are countless invisible stories still buried within World War II — that stories of ordinary children, for example, are a kind of light we do not typically see. Ultimately, the title is intended as a suggestion that we spend too much time focused on only a small slice of the spectrum of possibility.
______
It brought tears to my eyes.

It refers to the 'invisible' radio waves which connected Werner and Marie-Laure through their transmitters.