From Online Manual
A child board is a board which is located within another board -- the board it is in is called its parent board. Child boards can be used to organize discussions into several broad topics. Child boards are listed directly under their parents. in places such as the search page and the jump to boxes. For more information about child board settings, see SMF2.0:Boards#Settings. For more information about creating a child board, see SMF2.0:Boards#Modify_Boards.