If you're not familiar, Google's Accelerated Mobile Pages project aims to make the mobile web faster and less data-heavy. Websites that choose to support it offer AMP versions of their pages, which are then loaded instead of the full site. Google mobile search and the mobile Google+ site already load AMP pages when available, and now the latter's native Android app is joining in.
Links that support AMP will show a small lightning bolt icon, and upon tapping, will load the optimized version of the page. The normal version of the page can be loaded via a link at the top, if you so choose.
Keep in mind that this is different from Lite pages, which use Google's servers to optimize pages on the fly (whereas AMP pages are designed by the sites). Lite mode support has been added to Chrome, the Google search app, and Google News & Weather, but is only available in certain regions.