A buried session is a session that continues to run but is not a part of any window. If you have a long-running job that you want out of the way, it can be convenient to bury its session. It is used by default for the session where you initiate a tmux integration client using tmux Integration.
To bury the current session, select Session > Bury Session. To restore a session, select it from the list of buried sessions in Session > Buried Sessions.