It is part of the Terra Suite. It allows you to generate a terrain splatmap procedurally in the Unity3D Editor. Changes are done in real time. It uses the power of your GPU to apply your set of painting rules and in few milliseconds paints your entire landscape. This greatly improves your terrain creation workflow, both procedural and manual.
Using this tool you can paint everything, procedurally  or manually ( in next release ).

And you will get benefit using the powerful profile system that allows you to store your setting and use in other projects.

How can TerraPainter improve your terrain generation workflow:
– Works on any Unity terrain
– Multiple terrain tiles supported
– Ability to preview textures rules on terrain
– Splatmap rule presets
– Extendable texture rule database
– Realtime modifications of your splatmap directly from the editor
– Lots of settings to customize
– Fully integrable with other terrain tools
– It is GPU powered and can be used at runtime
– Compatible with Cascade River Bed Carving
– Compatible with Vegetation Studio SplatMap rule ( simply import the package in TerraPainter )
Upcoming Features
– Manual Painter with configurable brushes in combination with Procedural Painter
– Biomes Splatmap
Requires Shader Model 5.0