Google announced that as of the 21st April 2015 they will start to globally roll out the mobile friendly algorithm update. This update is basically going to start to take in to consideration if website is mobile friendly when it decides on where to rank it in the search engine result pages (SERP’s).
Google currently has an algorithm that it uses to determine which sites rank number one, two three and so on when a search is carried out on the search engine. This algorithm is highly confidential although they do give advice on how you can rank your site higher.
The algorithm is constantly changing and they often have big updates that will mean many sites literally fall 10 or 20 places, for example, overnight. When a large update is done by Google, they will explain briefly what the changes are and why they think they will help the end user who is searching on Google.