Under the Settings area for Forums you will find an Archiving option. If you have a very large site, it can be beneficial to reduce the size of the database table storing posts. Archiving topics is an alternative to deleting which allows them to still exist and be viewed as normal, but moves them to a separate database table. Some example options:
Archived topics cannot be posted in or edited, and will not show in search results. You can unarchive topics after they've been archived if you change your mind.
You can also optionally choose to store your archived posts in an external database.
This is a power-user option designed for communities with millions of posts and a long history where they would want to archive off old post data to conserve resources.