The Terrain Collider implements a collision surface with the same shape as the Terrain object it is attached to.
|Material||Reference to the Physics Material that determines how this Collider interacts with others.|
|Terrain Data||The terrain data.|
|Enable Tree Colliders||When selected Tree Colliders will be enabled.|
You should note that versions of Unity before 5.0 had a Smooth Sphere Collisions property for the Terrain Collider in order to improve interactions between terrains and spheres. This property is now obsolete since the smooth interaction is standard behaviour for the physics engine and there is no particular advantage in switching it off.