Network resources, such as web pages, commonly include online advertisements that are served by advertisement servers that are different than the parent web page (sometimes referred to as the publisher web page).  Advertisement creatives are written by many different companies with many different options.  As a result, some advertisements cause page loading to be slow and non-performant. 
AMP Ads for AMPHTML pages (A4A) is a way to render fast and performant ads in AMP pages.  An AMP page is an HTML page that has certain restrictions and features in an attempt to speed up regular page loading.  A4A advertisement creatives must obey a set of validation rules (with a limited set of allowed tags, capabilities, and extensions) in order to be rendered quickly and smoothly in the browser and do not degrade user experience.