From Online Manual
A child board is a board which is placed within another board. This effectively makes the board they are placed inside of become a parent board. Child boards can be very useful when you want to split up discussion from a board which covers a broad topic. It can also help keep things organized, when one subject is directly related to another.
There is no limit on how many boards you can make child boards, but it is a good idea to do it only when necessary as it can create confusion.
Child boards are shown directly under their parents in places such as the search page and the jump to boxes.
Administrators can make any board a child via the modify boards section of their Administration Center.