In simple terms, ads with landing pages that Google thinks to be most relevant to the query will be able to rank higher for lower cost-per-click bids.

This release comes after a limited scale trial held in Brazil, Spain and Portugal earlier this year where Google increased the weightage given to relevance and landing page quality in determining the Quality Score and how ads are ranked on Google. The aim of conducting these tests was first to see how these two factors impacted ad rankings on the Google SERP and secondly to improve the user experience with search ads because at the end of the day users appreciate.

The roll out process will extend over a few weeks during which many AdWords users will see variations in keyword Quality Scores and typical ad positions for their campaign. Eventually though things will stabilize and there shouldn't be any significant changes in the overall performance of  ad campaigns. Once the entire implementations process is completed Google will crawl the landing pages associated with every ad and determine its quality score.

Google's Jonathan Alferness says the reason behind giving more weightage to landing page quality is:

What we've seen is that there are ads available in the auction that is as good a quality as the top ads. But the landing pages - the merchant sites, the advertiser landing pages - are of much higher quality than the ads that we see at the top of our auction and this means the user experience isn't what it could be. In the end, we believe that this will result in better quality experience for the users.

What we always ask our advertisers to focus on is relevance - choose a landing page or site experience that is both relevant to the keywords that you're targeting and also a good experience for end users. This is just continuing to sort of push on those best practices. It gives us the ability to really reward those advertisers that have been doing this, whose landing pages really are some of the best in our systems.

Landing page quality has long been a factor in Google AdWords, but more as a negative signal. The new change will assign landing page quality a positive value, so advertisers should make sure the landing page's keywords and content are closely aligned with the keywords they're bidding on for their ad.

There are more details on this change in Google's announcement post here.