What is DNS query resolution ?
This is the Frequently Given Answer to that question.
Query resolution involves taking one or more partial or complete answers, retrieved from one or more content DNS servers, and combining them to form the final complete answer (that would have resulted had the DNS database been a single unified entity). It is the task of resolving proxy DNS servers.
Query resolution may involve combining complete answers. This occurs when content servers publish what they believe to be complete answers, because their databases hold complete information, but the data published are actually outside of the server's bailiwick in the eye of the beholder. In such cases query resolution involves chopping up the complete answers at the bailiwick boundaries to form what would effectively have been partial answers.