I keep seeing the word "quiet" in Sympa? What does it mean?

Explanation of the word "quiet" within the context of Sympa's UI

When you're inviting, deleting or editing subscriber addresses there is a default "confirmation" message that is sent to the address in question. By clicking the "Quiet" checkbox you suppress that message. It's very useful when making changes that list subscribers don't need to know about. It also may be confusing for your subscribers.