It's a utility class for rendering simple primitives like 3D lines, quads and cuboids, without having to keep track of manual objects. It is useful for debugging purposes, since it is quite a hassle to keep track of ManualObject instances, initializing them, attaching them to scene nodes and destroying them just for the sake of debugging. Using this class, all it takes is just one function call to draw a primitive. Note that it works like 2D rendering, e.g. it is cleared every frame. Thus, the draw functions need to be called every frame.
Currently, it only supports rendering of:
The Debug Drawing Utility Class is part of the official repository. You can download the latest source code from there.
A short introduction can be found in this forum topic or in the original Debug Drawing Utility Class article (slightly different)
Detailed usage example code you find on the "main page" Debug Drawing Utility Class. They are in C++, but it's easy to port them to C#.