In the Inspector window, click on the Overall Shape foldout to open it, you will see the properties as listed below:
Height Map: A grayscale map to determine terrain elevation (see below).
Tile X: Number of tiles on X-axis.
Tile Z: Number of tiles on Z-axis.
Vertex Spacing: distance between 2 vertices.
Base Height: height of the base (underground part)
Underground: should it generate the underground & ground thickness layer?
Enclose Volume: should it generate some extra triangles to close the terrain volume? Look at the terrain from the bottom up to see the effect.
The height of your terrain can be determined by a height map, which only contain grayscale pixels, where black is the lowest and white is the highest altitude. For example, we have this map, used in the Spring demo scene:
After sketching your height map, import it into Unity and set its importing options as follow:
Alpha Source: set to From Gray Scale
Read/Write Enable: set to True
Wrap Mode: set to Clamp
Then, assign it to the Height Map slot and see the result: