This is a list of images that are members of this category but aren't used in any English Wikipedia articles. An orphaned image is not used by any Wikipedia articles. It is possible, though, for an orphaned image to be linked from other namespaces, such as an editor's User: page, which is perfectly acceptable in most situations (but not for non-free images).
This list is generated once daily; the last generation occurred 19 hours 42 minutes ago. Whenever this page is loaded, the list is re-checked for orphan status. An image that has been de-orphaned will drop out of this list on refresh. However, before listing an image here for deletion, check to make sure it really should be removed. If an article is linked from a non-article space using no-display syntax (such as [[:File:Example.jpg]] - note the leading colon), it will be listed as an orphan here.
The fourth column links to edits made by the user immediately after they created the image. Normally, the first thing a user does after creating an image is insert it into an article. Therefore looking at that editor's subsequent edits often tells what article the image was originally intended for. When knowing the identity of a replacement image is useful (such as in a deletion discussion), this can help locate the replacement image, if one exists. If you click the link, the file creation is at the bottom of the list; each edit made after file creation is above it.
There is no category with the name "User-created public domain images from December 2006". If you believe this is an error, check the User-created public domain images from December 2006 page.