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.