This refers to what happened last month on Google. As of April 21st, all Google searches entered on mobile devices, began to favor mobile friendly websites. Google has adjusted their algorithm to be partial toward website properties their search engine sees as mobile friendly.
Sites judged as being too long to load, that are graphic heavy, hard to navigate and/or have too tiny of text will get less advantageous exposure (rankings) to mobile searchers.
The logic of this algorithmic alteration, as with all of Google’s changes, is to make it easier for searchers to obtain “relevant, high quality search results that are optimized for their devices.”