When you're driving on a very dark road, so dark that if you're headlights were to just go out, you wouldn't be able to see anything at all, let alone drive anymore. I think this is a metaphor for meeting someone so perfect that if you lost them you wouldn't find anyone better. She is his headlights on the dark road, but if she were to leave, he wouldn't be able to go on without her.
When you're driving on a very dark road, so dark that if you're headlights were to just go out, you wouldn't be able to see anything at all, let alone drive anymore. I think this is a metaphor for meeting someone so perfect that if you lost them you wouldn't find anyone better. She is his headlights on the dark road, but if she were to leave, he wouldn't be able to go on without her.