It’s a fact. Amit Singhal has talked about it in the New York Times that we believe there are some queries that deserve freshness. So, “QDF” was how he talked about it in the New York Times.
Query Deserve Freshness is a part of Googles algorithm which is designed for queries with a trend for a very fast change in the best results. If Google detects a very explosive growing for requirements in one field to that combination of words is assigned label QDF and all results in that field will be as fresh as possible.
by Matt Cutts - Google's Head of Search Quality Team