It describes the way a surface reflects light as a combination of the diffuse reflection of rough surfaces with the specular reflection of shiny surfaces.
Many neutral test cards are far from perfectly diffuse reflectors, and specular reflections can cause increased reflected-light meter readings that, if followed, would result in underexposure.
Using this configuration gloss was measured using a contrast method which subtracted the specular component from the total reflectance using a polarizing filter.
