He slept soundly all night.

Meaning: This sentence describes an action where a boy was in a state of rest with his eyes closed in a deep and peaceful manner for the entire night.

Choice of Words: Slept means to be in a state of rest with your eyes closed. Soundly means in a deep and peaceful manner. All night means for the entire night. So, "He slept soundly all night" means he was in a state of rest with his eyes closed in a deep and peaceful manner for the entire night.