In the Inspector window, click on the Surface foldout to open it, you will see the properties as listed below:
Tiling: Which tiling pattern to use on the surface? Quad or Hexagon?
Max Height: The surface maximum altitude.
Roughness: Should we add some randomness to the surface using Perlin noise?
Seed: a number used to generate random noise, different seed generate different noise pattern.
Note: Vertex displacement using Perlin noise is a multiply and un-normalized operation, that means it is clamped by the Height Map and noise value at a point will not change if you resize the terrain dimension (keep Roughness and Seed unchanged, of course)