The person she's singing to is afraid of dying and being left alone. the "shadows" that "almost killed your light" represent a previous brush with death. Taylor Swift has described this song as a "death lullaby" previously.
Taylor sings this lullaby to her partner, knowing that in the morning they will surely perish, but hopefully they will cross to the other side and be "safe and sound"; as opposed to alive and in the midst of the horrible battles outside.
She warns him yet again of the dangers taking place outside their door. She encourages him to listen to the soothing sounds of her lullaby, and remember it when they are passing to the other side.