The terrain toolset is (currently) two tools allowing for quick setup and fast iterating over terrain design whilst keeping a consistent looks.
First up is the terrain tool. Using input sculpts and masking objects the user can define areas for various types of terrain (currently included are: plains, hills and mountains). These masks can then be tweaked to fit the wanted look.
Besides this, splines can also be used to define rivers and roads to already carve those into the terrain.
If necessary, the user can also choose to export various feature maps (like debris, terrain types, slope and water) that can then be used in materials or other applications.
After that, the second tool handles the foliage placement allowing the placement of hundreds of user defined instanced trees.
With these tools you are able to set up a basic terrain that can be used for further development.