Presets
This button 
will open detail map presets.
While you can also use it with regular terrain handles, it is meant to be used for detail map and geoPaint.
To use it, just select a detail map handle and choose a map to use it as detailmap.
detailmap handle
Select a terrain surface and press the detail button 
to add a new detail handle.
Detail map is connected to material as Displacement map, select terrain
surface and press alt+3 to view displacement on your mesh, but to see its
real quality and detail I recommend using IPR render, if you are using Arnold
I recommend using regular lights to light your scene or just
press 
button to add two directional light quickly, and remove any extra
sample in your render setting, this way your render will be a lot faster and
without noise.
Detailmap falloff
Just select the detail map handle and press the falloff button 
.
The falloff may not be able to update properly in viewport, so I recommend
using IPR render to view and edit your falloff map.