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 inbox.
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
There are many types of bounces. Generally, bounces are divided in two categories: hard bounce (errors that start with 5**) and soft bounces (errors that start with 4**).
For more details, see this list of bounce types.
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.
Soft bounces are managed automatically and unsubscribed after N bounces. If you want more details on how this happens, please see this article from the MailUp online guide.
If you wish to, you can unsubscribe soft bounces manually, faster than what MailUp does automatically (this is typically not required). To do so, under Recipients > Channels > Email > Bounces, use the Recipient management feature (at the bottom of the page - view screenshot) and move them to Unsubscribed.