Impenetrable describes something that's impossible to get through. Whether it's a brick wall or a difficult reading passage, something impenetrable won't let you in.
Impenetrable comes from the Latin impenetrabilis, meaning "not to put or get into, enter into." Impenetrable, the adjective, has two meanings. When you're trying to learn a subject that is so complicated, so confusing or so detailed that it seems like it's in another language or from another planet, it's impenetrable — like calculus. Or a physical object can be impenetrable, meaning it is impossible to actually enter or get inside of, like that impenetrable nightclub that has bouncers watching every entrance to keep you and your friends out.