For questions related to Sitecore session-state configuration, usage, etc.

Session state is a way of identifying contacts on your website by assigning them a unique session ID and by using cookies. Every time a contact makes a request, such as triggering a campaign or a goal this information is held in session state until the end of session.

This means that fewer server trips are needed to write data back and forth to the collection database. Instead, xDB makes one trip at the start of a session to identify the contact and a second at the end to save all the session data back to the collection database.