Google rates the mobile version of your website / webapp.
For that, Google's using the mobile version of your website or webapp to index the content.
Since most of content in the world wide web is now accessed via mobile devices, it's more and more important to optimize your webcontent properly for mobile usage. Many providers promise a fully 100% mobile version of a website and tremendous SEO (search engine optimization). However, in most cases this turns out to be not as promised and inadequate. Graphics, images and other data are then usually not optimized for the respective device due to technical effort and thus meaning your page is automatically ranked backwards by Google, as it also takes longer to load your page. This and many other optimizations in the background guarantee that your page or web app performs superbly and is placed higher up on Google by default.
A strong performance of your website leading to a better indexing through Google and more organic search results of your site.
Google provides a tool which gives you the ability to test your existing site within a glance. Check it out.