What is light of my life, fire of my loins?

To describe the love of your life, or even more than that. Your world would turn into complete darkness or turn upside down without her.

It is in the opening sentence of Lolita the novel by Vladmir Nabokov / Lolita the movie, in which the narrator was obsessed with his stepdaughter which he named Lolita.

It is adopted by Lana Del Rey as lyrics in her song "Off to the races".