Meaning: This sentence describes a girl sleeping peacefully throughout the night. It's like saying,
Choice of Words: "Slept" means to be in a state of rest with your eyes closed. "Soundly" means deeply and peacefully. "Through" means continuing until the end of a period of time. "The night" is the period of darkness between sunset and sunrise. When you put them together, you get a picture of a girl sleeping peacefully all night long.