From Online Manual
A child board (or sub-board) is a board which is placed within another board. The board it is placed in is the parent board. Administrators use modify boards to set this up. Child boards can be used to organize information and discussions on a forum.
Child boards are shown directly under their parents in places such as the search page and the jump to boxes.
Parent board statistics (number of posts, last post) are not affected by the posts in sub-boards of its sub-boards.