Quoting from the AMP website "The Accelerated Mobile Pages (AMP) Project is an open source initiative that embodies the vision that publishers can create mobile optimized content once and have it load instantly".
Although driven by Google, this is an open source collaboration across several various partners. What it does is either strip away or move code that slows down the building of the page when you go to it on your phone or tablet. This also has the positive effect of potentially moving your site up the search engines (remember mobilegeddon?).
For system like WordPress, they have produced a plugin. Sadly, other systems are slow to follow e.g. Joomla has nothing native, but one of their extension developers, Weeblr, have just launched something.
Whatever system you use, take a look at implementing this, especially if, like most of the rest of the web, more people are looking at your site on mobile devices. Yes, it may mean more work and some extra cost, but, it should be worth it in the longer run!
For the lowdown, direct from the horses mouth, visit here.