An exit is the last page view in a users session. We capture each user page exit but also roll it up to a site, site collection, and hub level as well.
Exits are related to bounces because a bounce is a when someone exits the page through the same way they entered. Basically a bounce is a specific type of exit.
Exits are important to measure because they indicate that users stopped their session with your content. This could be good or bad depending on the view duration and purpose of the page. For example if the page is informational or the end of a process then you would expect or even hope that the page has high exits, suggesting people found what they were looking for.
High exits in navigational pages can be seen as a bad thing as you would expect users to use them to move from your landing area to an informational page. So if you see high exits or a high exit rate on your hub pages then consider improving the navigation experience. Users could be missing buttons hard to find areas or are not finding content they are looking for.