When your email will be rejected by the recipient server due to following reason that it will be bounced back to send server (to you in this case) hence we called them bounced IDs.
Reasons could be any things such as email address not valid or a temporary issue or your email content are smees as spam etc.
There are 3 types of Bounce categories we have and you can read about them as follows.
What are the different types of bounces?
When any user does not want to receive any further email from your end then they can opt-out or unsubscribe themselves from your mailing with this Unsubscribe link given in your email, so they will not receive any further email next time onwards.
Kindly Note: You must have to give the unsubscribe link to in your email to get these statistics.
Read more about How to add unsubscribe link?