This tutorial is basically a follow up to the Radiance Patterns tutorial and details how normal maps can be used to specify Radiance textures. Textures in Radiance terminology is a perturbation to the surface normal to give the impression that the surface has detailed physical features. If for example a point on a surface has its normal perturbed towards a light source the point will receive more light than if the surface normal is perturbed away.
Although, like Radiance patterns, textures are not often necessary for numerical lighting simulation, and indeed are ignored if they are on an illuminance sensing surface, they can provide extra realism to visual Radiance renders and there are certain circumstances where they may be useful numerically and/or save time by not requiring the creation of detailed physical geometry.
The video below details the process.