AMP stands for Accelerated Mobile Pages, Google themselves say that:
"AMP is web component framework that you can use to easily create user-first websites, stories, emails, and ads."
AMP was introduced in 2016, in part as a direct response to Facebook and Apple initiatives. It is intended to provide an optimised loading experience for viewers on mobile devices. At a basic level it requires webmasters to re-author a page using Google's framework, and hosting the content on a Google domain.
AMP has been controversial since it was introduced. The requirement to publish content in a Google approved manner, and host on Google servers is contrary to the principals of an open web. Google also ranked AMP authored pages higher in its search rankings and news stories, and marked them in search results as faster loading.
Google touts higher engagement and click through rates for AMP pages, but some of this may be down to the preferential treatment in its search results.
As part of its June 2021 'page experience update' Google is dropping its preference for AMP authored pages.