Bounces can be soft or hard. These are email addresses to which email delivery has failed. This can happen if the receiving server is busy or is not responding, or if the email address is invalid (or doesn't exist).
The system does try to resend the emails (in case the servers our temporarily busy), and stops only after a few tries.
Since soft and hard bounces affect the reputation of our sending IPs which in turn affects email delivery, these are added to the Global Suppression List so that no emails are sent to these addresses in the future.