A good way to gain an understanding of pbrt is to follow the process of computing the radiance value for a single ray in a debugger. Build a version of pbrt with debugging symbols and set up your debugger to run pbrt with the killeroo-simple.pbrt scene from the scenes/ directory. Set breakpoints in the SamplerRenderer::Render() and SamplerRendererTask::Run() methods and trace through the process of how a ray is generated, how its radiance value is computed, and how its contribution is added to the image. The first time you do this, you may want to specify that only a single thread of execution should be used by providing --ncores 1 as command-line arguments to pbrt; doing so ensures that all computation is done in the main processing thread, which may make it easier to understand what is going on, depending on how easy your debugger makes it to step through the program when it is running multiple threads. As you gain more understanding about the details of the system later in the book, repeat this process and trace through particular parts of the system more carefully.