Opaqueness is the characteristic of being either difficult to see through or hard to understand. The opaqueness of a passing car's windows means that you can't see who's driving, while the opaqueness of a poem makes it hard to figure out the poem's meaning.
When light can't shine through an object, or can just partially penetrate it, that's due to opaqueness, which you can also call opacity. When something is obscure or impossible to comprehend, that's another type of opaqueness. The opaqueness of that 1,000-page novel you've been trying to slog through might require you to keep looking words up in the dictionary as you go.