Terrain detail is handled in a similar way to tree distribution, allowing you to automatically add grass and small plants to your environment based on customisable surface based rules.
Detail meshes can be made for you from a texture using a simple quad mesh, which are called billboards. However, you can also add custom low poly meshes as detail objects.
You have the option of adding detail meshes that react to the wind, or static detail meshes such as rocks.
This gives you the flexibility to have farmland full of crops, while having a forest full of bushes, and a beach littered with pebbles.
Some Unity terrain settings pertaining to detail rendering, such as rendering distance, are also exposed on the Detail panel, which is useful for making global changes across all your terrains at once.