A "phantom" artifact is an artifact whose hash named appears in some artifact but whose content is unknown. For example, if a manifest references a particular SHA3 hash of a file, but that SHA3 hash is not on the shunning list and is not in the database, then the file is a phantom. We know it exists, but we do not know its content.
Whenever a sync occurs, both each party looks at its phantom list and for every phantom that is not also marked private, it asks the other party to send it the content. This mechanism helps keep all repositories synced up.
This page is similar to the /bloblist page in that it lists artifacts. But this page is a special case in that it only shows phantoms that are not private. In other words, this page shows all phantoms that generate extra network traffic on every sync request.