Unlike the joints or bones that are imported in the FBX file as part of the rigged model, Unity's joints can be used to connect game objects that respond to physics within the game.
Create a new scene by navigating to File | New Scene....
Add a plane game object using the menu bar by navigating to GameObject | 3D Object | Plane.
Click on the Plane in the Scene view or Project panel to adjust its parameters in the Inspector panel.
The plane game object should be centered within the scene by default.
Rename the plane
groundPlaneby typing this into the field at the top of the Inspector...