The way AMP (Accelerated Mobile Pages) works is, you make a special version of your web page that behaves itself. Just a normal web page with some extra tags added, and a few bad things (like slow ads) removed.
When Google comes along to spider your site, it notices that you put in a link to that well-behaved page. It saves a copy of the whole thing, pictures and all.
Now Google has that copy at the ready, and should someone on a smartphone request it, Google can pop the whole thing up—words, images, scripts—in less than a second, even on a slowish network. It’s nice.
There’s nothing inherently ‘mobile’ about AMP. AMP is designed to be mobile friendly, and with slow hardware and high latency connections, the boost you get with AMP on smartphones is going to be felt a lot stronger than on desktops. But AMP isn’t mobile only – it’s mobile first.