The Google algorithm is constantly changing, and every change reshuffles search results to some extent. But this most recent change will alter the results for 35% of queries, making it one of the largest updates in recent years. Essentially, though fresh content has always been critical to search ranking performance, Google has updated its algorithm to give more significance to the most recent content created for a specific topic. This doesn't mean that newer content is always going to outrank older content, or even that newer content will tend to outrank older content. But Google has clearly concluded that in many cases users will be better served by seeing more recent information on a topic rather than older information, and the search engine has adjusted it's ranking factors to account for this fact.
Under the algorithm update, “freshness” is determined by a number of factors, most of which Google has not disclosed. However, it seems Google doesn’t consider new content added to an old page to be “fresh.” This means adding info to an article that was published months ago won't give it a “freshness boost.”
Barry Schwartz, news editor for SearchEngineLand, has written a detailed article about this update. Read it here.