Some of the data that Google uses to improve their search algorithm comes from the feedback of thousands of search quality raters. These raters conduct actual searches and rate the top quality pages according to a publicly available set of guidelines.
Data from these quality raters don't directly impact search rankings; instead, Google uses their ratings to inform the search algorithm. Nevertheless, it's good for content creators to know some of the main things that these raters look for. Google's update of the guidelines last week provide insight into the most important things to consider when creating search-friendly content.