Opacity is a property of a prim that can be set through the edit window -> Texture tab or through script. It adjusts the level of transparency an object has (how much it is "see-through"). Also sometimes called alpha. An object can not be transparent and shiny at the same time. However in 1.9.1 they could be, before most of the new shader code was removed, this means in the future transparent + shiny may be possible.