In February this year, Google launched a significant algorithm change, which has also become known as Panda. The change is aimed at identifying low-quality websites and making sure they do not rank too high when people search on keyword terms. The idea is to weed out websites known as content farms that rank well but may not provide the best user experience.
Ever since this algorithm change was made some sites have seen a drop in ranking. However the sites with quality original content have seen their ranking improve. Google wants to see more sites of higher content quality to give the user a good experience so they continue to use their search engine.
Google employs many metrics to determine the quality of a website. It looks at the time people spend on the site. They look at bounce rate, which is a measure of the percentage of people that leave a site without doing anything. They look at social signals such as shares and +1’s as a way to see if people are recommending a given webpage. They also look at page views per visit as a way to see how people are navigating through a given site. If your site has suddenly dropped in ranking then it’s time to focus on these areas mentioned.
Unique content has always been important but Google now wants unique content that is also high quality content. So make sure your text, blogs and product descriptions are original and useful to the reader - does it answer the search query better than other pages on the web. Other things that will help are making sure visitors can easily find their way around your site, make it obvious what each page is about. If the content is engaging then people will also share the article on the social media platforms which will in turn increase traffic and gain you some inbound links.
If you want more guidance on this take a look at what Google has written here http://googlewebmastercentral.blogspot.co.uk/2011/05/more-guidance-on-building-high-quality.html