Query Deserve Freshness or “QDF” relates to the freshness of search results. More specifically associated with Google Algorithms for search terms or search queries on specific terms and is effected by the ever changing Daily News and/or Google News. Search Terms that normally lay dormant of the same suddenly become popular and the search results pick up on this to dynamically adjust the search results. This is NOT a Google Update and should only effect a small amount of queries and sites for a particular time frame.
For example if “Pet Rock” is no longer a popular search term, then suddenly it becomes a big new flash and hot topic, you would see drastic search result changing in the algorithms.