The page URL is a parameter passed by Google's Publisher Tag (formerly Doubleclick for Publish) to Google Ads.

Since adverts are rendered in an iframe, GPT may not be able to determine the URL of the page. If you are using Ad Exchange or AdSense to fill impressions, these systems can't send contextual information about the page to buyers without a page URL.

Article Page

When an inline MPU or inline top banner advert appears on an article page we pass the Canonical URL if set in Express.

If the canonical URL is set in the HTML markup we pass it. If not set, we don't pass anything for the Page URL. 

You can set any meta field as the canonical URL meta field in Pugpig Settings under the Post Canonical URL meta field heading. This allows for easier mapping of the canonical URL from importers.

Timeline

When using a Google Ad card as a timeline MPU we pass the following:

1. The canonical url of the timeline, if it exists (website equivalent page of the timeline)
2. The nearest surrounding post for a canonical URL. This can be up to 2 posts in either direction of the ad.

Google's Ad Manager Crawler bot must be allowed to crawl the domain of your website.

Further Reading

Page URL was an original Ad Sense parameter. Ad Sense was originally website only. When Google bought Doubleclick for Publish certain Ad Sense parameters were integrated into DfP. See AdSense Attributes

Page URL is not mentioned in the docs except in passing. Google Publisher Tag now infers the URL of the page itself. However in Bolt's case the app this URL is local. So we explicitly set the value. See Specify Page URL.