In simple terms, a bounce is an email delivery error. These types of errors (i.e. bounces) occur when your email fails to be delivered to your recipient's inbox for a variety of reasons, such as an incorrect recipient email address or a temporary error such as a full mailbox.
Managing these bounces is an important step in keeping your mailing list optimal. If the email that you sent out (or part of your email campaign) bounces, you should consider carefully looking at the email address(es) that bounced and act accordingly. The key is that you want to minimize the number of email errors (i.e. bounces) in your mailing list.
The bounces section is found under Recipients > Channels > Email > Bounces
In detail, there are thousands of email bounce types. Generally, bounces are divided in two categories: hard bounce (errors that start with 5**) and soft bounces (errors that start with 4**).
For a complete list of bounces, view this knowledge base table.
All email addresses that result in a hard bounce are automatically unsubscribed from your mailing lists. Those email addresses can be found under Recipients > Channels > Email > Unsubscribed.
For soft bounces, and depending on the type of bounce, you should decide to unsubscribe or not a given email address.
Under Recipients > Channels > Email > Bounces, the recommended method to unsubscribe all the bounced email addresses is to use the Recipient management feature (at the bottom of the page - view screenshot) and move them to Unsubscribed.