A person is sleeping, but without a blanket. Or their blanket slipped off, got tugged all the way down to their feet, etc. Someone comes along and covers the sleeper with their blanket. Look how affectionate they are, they worry about the sleeping comfort of the sleeping person! ("Needs A Better Title")
Example: I've seen this lots of times in Movies and TV, but the most recent occurrence (triggering the impules to search for a trope) was Donnie Brasco. Scene: One of the few times Donnie is home, he finds one of his daughters asleep, sans-blanket, and he pulls the blanket back over her body.