Cumulative Layout Shift measures the total amount of user-visible layout shift using the Layout Instability API by summing the layout shift score for all layout shifts where the ad iframe shifted or caused the layout shift.
This metric is not indicative of a specific issue. It should be used to help identify areas for improvement and track that improvement over time.
To avoid layout shift, it's important to set sizes for ad elements before the ad tag loads. This can be difficult or impossible to do perfectly—especially for ad slots that accept dynamically sized ads—but there are best practices you can follow to minimize layout shift in most cases. For more information, see the guide to minimizing layout shift at the GPT developer site.