A parked domain is one whose nameservers belong to a parking or for-sale service — Sedo, ParkingCrew, Bodis, Afternic, Dan and their kind — so nobody runs a website or mailboxes there. The registration is held for sale, for a lapsed project, or for the advertising a parking page earns. Most parked domains accept no mail at all; the ones that do are collecting it rather than delivering it to anyone who signed up.
What Spaw answers
The field is parked, on both the address and the domain answer. It is read from the domain's NS records against a curated list of parking nameservers, and only once the domain has passed the mail-server check, so it is null on a domain with no MX record — that domain already answers no_mx_records. A parked domain that still accepts mail answers the verdict risky with the reason parked_domain and carries +60 on the score.
What it does not mean
A registrar's default DNS is never on the list, because real sites run on it too; a domain a small company keeps on its registrar's own nameservers is not parked. And parked is a fact about the domain today, not about the address: a domain that was a real company and is parked now has lost its people, which is why the answer is to ask for another address rather than to keep sending.