Read the info here:
Please note that if you are a scenery author and not a programmer, this will not directly affect you at all. (But see the section on compatibility - old plugins will still work!)
Version 9 will also feature a lot of new datarefs - this should make things a lot easier for authors doing animation.
The new API will allow programmers to access the terrain mesh. This means that add-ons that want to add dynamic graphics content by drawing (e.g. drawing in baggage trucks) will be able to place that content "on the ground" no matter what the surface of the airport area. I think this will allow for some interesting new add-ons.