the core is a solver of the rendering equation. It solves the equation by using the Monte Carlo method. The equation is basically "color of a pixel is an integral of all light path contributions". Integral means sum of infinite number of infinitely small parts (light paths). Monte Carlo works by randomly selecting some samples (light paths), calculating their contribution (BRDF sampling, light sampling, ray tracing), and averaging the results. If a highly contributing path is randomly selected, the pixel is bright, if the path is occluded, the pixel is dark (random noise). With more samples added, multiple random bright and random dark paths average to the correct solution.