This is the 4557th most frequent German word.
ruhen
"Ruhen" means "to rest" or "to be calm/still."
In this case, 'ruhen' relates to silence or calmness, indicating that the voices ceased when the teacher arrived.
Die Stimmen der Kinder ruhten, als der Lehrer den Raum betrat.
The voices of the children rested as the teacher entered the room.
The word 'ruhen' in this sentence indicates the act of resting physically after exertion. 'Sich ausruhen' is a reflexive verb form commonly used in this context for recovering or relaxing.
Wir ruhen uns nach einer langen Wanderung aus.
We rest after a long hike.
Here, 'ruhen' conveys a metaphorical sense of being untouched or inactive, describing the inert state of the book within the box.