It's a French phrase that is often used to describe a voice that is:
* Clear and unadorned - free from any vocal embellishments or artificiality.
* Natural and unforced - sounding effortless and genuine.
* Beautiful and resonant - pleasing to the ear and carrying a strong emotional impact.
It can also be used metaphorically to describe something that is pure and authentic in its expression, like a piece of art or a piece of writing.