It is a lot like how Google and event Bing users previous search but as an extended period of time.
I have a more detailed write up on this at Search Engine Land where they describe "any content available in an individual’s Search History can be used to personalize the search experience. This is currently limited to the past 28 days if the individual is not signed in to Bing, or 18 months if they are."
Here is a video from Bing on this topic:
Forum discussion at Search Engine Roundtable Forums.