Yes, this is true..if you have quality content on your site that is relevant and useful for the users, Google will rank you higher for a fixed period of time but gradually if the document is unable to get natural links pointing to it, it will decline in rankings. Hence, links are the biggest ranking factor for Google. This has been the case for many years but recently there were discussions that Google might value quality content more. But, the fact is content+links=rankings.
Have a look at the webmasters video:
Also See:
