Material is a property of a prim that can be set through the edit window or via script. Material influences the friction of a prim as well as the default sound it will emit on collision. The types of material are (in order of friction from least to most): light (deprecated), glass, metal, plastic, wood (default), rubber, stone, and flesh. An avatar's material can not be changed and is most likely "flesh" (or something similar if it has one at all).