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".