Quality Score is a supposedly mysterious number that was pioneered by Google to “rate” or “evaluate” how well you are doing as an advertiser, but other search engines are now using their own versions of quality score for the same purpose. I don’t remember who said it, but Quality Score is Google’s way of saying that you should or shouldn’t be advertising on specific keywords or specific websites.
I say that quality score is “supposedly mysterious” because, in fact, it is pretty straightforward. In the beginning of the AdWords system, there was no such thing as quality score — there was only click-through rate (CTR). Google wants people to click on the ads that they run, so they favor advertisers who have high CTRs. High CTRs for advertisers means high revenue for Google. But as Google has grown more and more sophisticated, they have created a very advanced system for measuring quality that goes way beyond quality score. You need to hit those other factors. But at the end of the day, if you only get one thing right, you should nail CTR, and if you do that, your quality score will improve.