Google Ads Quality Score

The Quality Score is Google’s evaluation of the overall user experience that your advertising and landing pages give when users search for your keyword(s).

The Quality Score is based on the combined performance of three components:

Expected clickthrough rate (CTR): The probability that your ad will be clicked when it is displayed.
Ad relevance: The degree to which your ad fits the intent of a user’s search.
Landing page experience: The relevance and usefulness of your landing page to those who click on your ad.

Quality Score is not a key performance indicator and should not be optimized or aggregated with the rest of your data.
Quality Score is not an input in the ad auction. It’s a diagnostic tool to identify how ads that show for specific keywords affect the user experience.

[Source: Google Ads Help]