Every post can have a parent. Any post that has a parent will not show up in the linkTo("main listing", "post#index") ?>. However, the post will appear again if a user does any kind of tag search. This makes it useful for things like duplicates.
Please do not use parent/children for pages of a manga or doujinshi. It's better to use a linkTo("pool", "#pools") ?> for these.
To use this field, simply enter the id number of the parent post when you upload or edit a post. To search for all the children of a parent post, you can do a tag search for post:nnnn, where nnnn is the id number of the parent post.