It's a four letter domain name, for example ABCD.com
You may also see something like this one day: CVCV.com (Consonant Vowel Consonant Vowel) or VNV.com (Vowel Number Vowel)
Hope this helps.
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Only one I have left at present is QVDF.com which could stand for QualityValueDomainForum, offers start at $50 for this one registrar EuroDNS 31 Oct 08 expiry.