Reason behind Opaque translucent transparent
Opacity or transparence is a matter of light reflection.



The way light is reflected or absorbed depends on severals parameters, first the composition of your material. If it contains molecules or crystals absorbing the visible light, you'll see it black, or colored with the color corresponding to the wavelenght the less absorbed. 

There is also crystallographic considerations to take care of. If your material structure is well ordered, dimension of your latice will be set so to reflect the light, then you'll see it transluscent or opaque. For something without ordered structure, you'll probably see it transparent if the visible light isn't absorbed. We'll speak about "amorphous" material then like glass for instance.

edit : if you feel easy with those concepts, I can go further in the explainations, but since you asked for a really vulgarized answer I haven't developed more.