The word "Ordial" is probably related to "ordinal," or anything of a specified position in a numbered series. It gets its name because it fills a perceived hole in the order of the multiverse, not because of anything known about its specific nature.
The notion is that the Ethereal Plane joins the Inner Planes with the Prime+Material, and the Astral Plane joins the Prime+Material with the Outer Planes, so - according to the Unity of Rings - another plane should complete the cycle by joining the Outer Planes directly to the Inner Planes. This would also satisfy the Rule+of+Threes by providing a third linking Transitive Plane - the Plane+of+Shadow not being considered a transitive plane at the time, and anyway not joining any major parts of the Great Wheel together in a vital way.
The problem is, if such a plane exists, why don't planewalkers know about it? And what does it do?
Some of these issues are discussed - and new questions are raised - at http://mimir.net/mapinfinity/ordial.html, http://mimir.net/mapinfinity/dragons.html, and http://mimir.net/mapinfinity/burbank.html.
The Ordial Plane cannot be considered "official," but it's potentially a very intriguing concept. It may be considered similar to the "Macrocosm" - a theoretical plane related to the Chososion monster.