Google officially launched its Accelerated Mobile Pages (AMP) in February 2016 to be lightweight versions of source websites that used a lot less bandwidth. But some webmasters have deliberately included less content in their AMP editions with a link to the original page, and Google isn't happy. By next February, AMP pages and their source counterparts must have identical material, the search giant declared.
Sometimes, the lightweight AMP version cuts out ads and links around to other parts of the source page's website -- hence the motivation to provide 'teaser' content that users must click through to see the full page. But Google sees this bait-and-switching as infringing on AMP's mission to 'dramatically improve the performance of the web and deliver a fast, consistent content consumption experience,' as the search giant's blog post put it.
Google assured that, after the February 2018 deadline, should the lightweight AMP version be incomplete compared to the source material, a link will redirect visitors to the original page. This won't affect those pages' Search ranking, and Google will invite those webmasters to fix the shortened edition.