If a postcard is unaddressed, it means you forgot to write the recipient's mailing address on it. But if an issue or question is unaddressed, it hasn't been dealt with.
When a politician speaks in public and manages to avoid answering the most important questions reporters ask, those issues remain unaddressed. And if you stay after school for extra math help but leave still not understanding what a polynomial is, your questions are unaddressed. This meaning of unaddressed comes from the verb address in the sense of "acknowledge and deal with something."