It's a poetic and evocative phrase that could be interpreted in several ways depending on the context. It could refer to:
* The quiet, contemplative nature of souls: The souls that remain silent, observing and reflecting.
* The spirits of the deceased: The souls who have passed on and are now silent.
* A hidden, secret world: A place where souls are quiet and unseen, perhaps a place of peace or mystery.
Without more context, it's impossible to know the precise meaning intended.