The Open World Tools in Unreal Engine 4 allow you to place Static Meshes inside your levels using a computer algorithm to determine placement instead having to do this work manually.
This enables you to quickly generate a natural looking forest comprised of many different species of trees, plants and bushes.
When talking about the Open World tools we are actually referring to the two different tool sets inside of UE4.
The tool set that you use depends on what you want to accomplish.
For example if you want to make an expansive pine and birch tree forest then you're going to want to use the Procedural Foliage Tool.
If you want to densely populate the Landscape Terrain used in that forest with grass, flowers, and small rocks, then you would use the Grass Tool.