Colored borders?

What do the green/yellow borders on thumbnails mean, respectively?

yellow = child
green = parent
red = flagged for deletion

blue = waiting for approval

what does it mean by child and parent?
and what does the approval do?

A parent node is the "head" of a group of related posts, all the others are children. The parent should be the original, best, or first page of the set of related posts. The parent post is the only one that shows up in the default 'posts' listing, the children are hidden by default, but will show up if you search for their tags.

Moderator approval is required for all posts made by unprivileged members. Without moderator approval, the posts will be deleted after three days.

thanks