Why some materials are transparent, but other ones aren't?
A brief summary: It is caused by the electrons in the particles in the substance. When electrons are orbiting the nucleus of an atom they can move to higher and lower energy levels. They do, however, need energy in the form of photons to do this, this means that when a ray of light hits the atom, the electrons in it will absorb some of the energy from the light ray. It needs a specific amount of energy to move up to the next energy level so will only absorb light from a specific part of the spectrum. This means that in some materials most of the light can pass through it and it may only absorb light waves not of visible light. 

So wood, to some frequencies of light, is see through but not to the light waves we can see. That is why we can have mobile and radio signal from inside houses but only see through glass.

(Electrons are actually much more complicated and theoretical than this suggests but this)

Try watching this:  https://www.youtube.com/watch?v=Omr0JNyDBI0