What exactly dropped domain is?

if i buy a going-to-expired domain from godaddy's auction (let say an hour before they're expired and returned to the registry - cost $5 in buy now button), does it mean that I buy a dropped domain or still a normal one?

It is a dropped domain.

If you want to retain its traffic and pagerank, try to re-create the site (use waybackmachine.com). But it's not a guaranteed result though. I once bought a dropped domain with a PR of 3, after 3 weeks, google dropped it to PR0