The query might return a list of selected items in a shopping cart, or posts in active forums threads, or whatever your web application needs to retrieve from your database.
active forums is functioning as an adjective here. It is obvious, of course. But what I can't get is why does it end in s making it plural? Could it be written like active forum threads? If so, what would the difference be? And I have certainly many times come across nouns ending in s used as adjectives but never really understood the reasons why there was a particular need to make them plural instead of leaving them in their singular form.