By "unlisted" it means available only to those who know a dedicated URL, meaning no public display, no channel availability, and no search engine results. It also means access to anyone you choose, not just someone with a YouTube account.
While Mashable speculates this option will be very valuable for education purposes and for non-profits, it's hard to believe it wouldn't also be useful for services looking to distribute video selectively to certain audiences as a premium or exclusive offering.