This command sets the opacity of the current material. In other words, how transparent the material is, or how much you can see through the material.
value is a range of 0.0 to 1.0.
opacity 1 # totally opaque and the default for any surface.
opacity 0 # totally invisible, useful for making collision boxes.
opacity .5 # the object can be seen as well as seen through. Used for simulation of glass