Private boards are like homes for each member. All members have their own private board which are password protect. Members get their private boards some time after their registration because it may take sometime for creating the board. As soon as a private board is created it's owner is sent a message with details of board. A member may share his password to private board with his friends.
Normally a private boards is meant for drafting stories and all, or for a private talking with friends or keeping a diary and such kind of things.