The Build tool is used for creating final terrain object which is optimized for runtime performance. To enable Build tool, click on the Build tab in the Inspector or press F7:
• Chunk Size: Size of each terrain chunk, choose an appropriate value to take advantage of LODs and light map baking.
• Mark Static: Should it mark the terrain and its child as static for baking?
• Generate LOD: Should it generate a lower resolution of this terrain to reduce rendering overhead?
• LOD Transition: Transition between LODs.
After hitting Build button, the final version of the terrain will be created as child game objects. To continue editing, you should click on the Fix button on top of the Inspector GUI: