All it does is move topics/replies from the current tables to the archive tables - speeding up access to topics which are still being read/used.
To use it, simply open a forum, then click the "archive the forum" button. You'll get an option on where to set the cut-off (how many weeks/months, etc.) of current topics to keep listed. Any topics older than the cut-off date will be moved to the archive tables and not show up in searches, etc., unless the "archived posts" box is checked during a search.
Search is your friend “I was having a mildly paranoid day, mostly due to the fact that the mad priest lady from over the river had taken to nailing weasels to my front door again.”