QS is a score the system uses to assign grades at the keywords level, as a feedback of how good/poor you're doing in the value chain, search query triggered by the keyword > your ad > your landing page. The 3 metric that you'll need to monitor are: - Expected CTR - Ad Relevancy - Landing Page Experience Each of those can be: below average, average, or above average. The principle is that Google is defending its search engine's UX. If the searcher is searching for white elephant, and you advertise him with a blue monkey, for Google this is depleting the searcher UX, and you get "fined": your score will drop down, and your CPC will go up.