All brand-new user accounts are in a moderated state until a moderator approves that user's first post -- it keeps spam posts from spambots from being visible on the forum at all. Once a moderator approves a new user's first post, that post becomes visible to everyone and the new user account then has full access to the forum and can post without further moderation.
The forum already auto-blocks spambots listed on stopforumspam.com, but new spambots still can sneak in sometimes, which is where the auto-first-post-moderation lets the staff see and ban those bots before their spam is ever visible to non-staff members of the forum.