These tables store the unread data for phpBB and they can become bloated as users do not use the Mark Read buttons. If you have a larger board, this can consume a lot of your database space unnecessarily. You can run the following queries to mark every topic and forum older than one month read for all users. Topics within the last month, retain their existing status.

DELETE FROM phpbb_topics_track WHERE mark_time< UNIX_TIMESTAMP(DATE_SUB(CURDATE(), INTERVAL 1 MONTH));
DELETE FROM phpbb_forums_track WHERE mark_time< UNIX_TIMESTAMP(DATE_SUB(CURDATE(), INTERVAL 1 MONTH));
UPDATE phpbb_users SET user_lastmark=UNIX_TIMESTAMP(DATE_SUB(CURDATE(), INTERVAL 1 MONTH)) WHERE user_lastmark<UNIX_TIMESTAMP(DATE_SUB(CURDATE(), INTERVAL 1 MONTH));