The Toolbox window helps with rapid object spawning, level designing and advanced scene editing. It contains several modes, each contained in a separate tab. You can switch between these modes by clicking on their corresponding icon (plus icon, brush icon, etc.).
The default and most used toolbox mode is Spawning. In this mode you can add new actors to the scene by dragging them into the viewport or scene window. Actor types are grouped into categories so you can quickly find the desired object.
The vertex painting tab supports painting a selected model instance with a color. To learn more about this tool see the related documentation here
The foliage editing tab supports different modes for foliage editing including spawning, removing, and managing tools. To learn more about foliage see the related documentation here
The terrain carving tab supports different modes for terrain editing including carving, painting and managing tools. To learn more about terrain see the related documentation here