When you draw an object, how do you make it look smooth and shiny? Soft and furry? Rough and pebbly? Depicting textures convincingly in your drawings involves finding a way to translate your sense of touch into visual terms. A good way to start is to gain an understanding of how we are able to see and distinguish among textures in the physical world, and how those visual clues convey tactile qualities.
Light that falls on a rough object is dispersed in many directions. Light that falls on a smooth object bounces off it in a single direction and is seen on the object's surface as a distorted picture of the fight source. Reflections help define how smooth a surface is.