UPDATE: TripAdvisor's Martin Verdon-Roe has responded directly to Tnooz, regarding what this means for TripAdvisor moving forward.

It means that not only are our ads more trustworthy but the performance for the clients has also improved. CTR is up and we have also implemented a tracking technology for “viewability” and that is not showing that the Ads below the fold are now just as valuable as those above. In fact the ones below are now producing a stronger viewability metric compared to the 728x90 which is at the top of the page.
This then leads us down new avenues and opportunities. E.g. can we find areas below the fold with high user dwell time. We can put advertising placements there in the confidence that we will get great user exposures but only when the users actually get there. This helps the advertisers performance and allows the sales team to find new impressions on TA that otherwise would have been unsellable.
Online ads are oftentimes the land of the unknown: what exactly is an impression? If a user lands on a page and doesn't scroll down to see any of the ads, do these count as impressions?
If so, should an advertiser pay for them even though the user did not actually see the paid ad?
TripAdvisor has addressed this issue head-on with a new product called a "Delayed Ad Call." Ads on the site will now only register as an impression when the user scrolls down to the on-page area where the ad appears.
IAB generally considers an impression as an ad that stays at least 50% in view for at least one second. While this makes sense, its clearly not enough time for the average human to even register an ad - not to mention actually convert from a passive viewer to an active purchaser.
In addition, a recent study by AdSafe Media revealed just how limited the impression metric can be: 50% of ads bought directly on Publisher sites were never seen by the user, and 60% of ads on Networks and exchanges were never seen by the intended user.
Martin Verdon-Roe, TripAdvisor's VP Global Display Sales, speaks to this developing understanding of display advertising.

Our clients are always looking for measurable results and confidence in the media they are buying, whether that be for a Branding or a ROI campaign. With our delayed ad call in place advertisers can now be fully confident that the results that they are measuring on TripAdvisor are a real and true representation of their media investment.
By reducing the "fake" impressions generated by ads that are never seen below-the-fold, TripAdvisor can both enhance the value of the advertising product and more accurately deliver metrics to advertisers. This increases trust and provides more useful metrics for advertisers seeking to optimize their ad copy.
A spokesperson for IAB UK lauded the move in the announcement:

The IAB (UK) has formed a cross-industry group of experts from the buy and sell-side to review Viewable Impressions and discuss and surface the opportunities and challenges of such a move. TripAdvisor’s proactive move to counting an impression only when the ad comes into view is a bold market-leading position in advance of industry consensus in the UK.
With such a large publisher delivering an updated ad experience, other travel publishers should take note. This has the potential to change the game, as far as ads are measured and paid for.