Google are rolling out a major update - some say as big as the 'Penguin' update 10 years ago. It's described as an attempt to reward those sites that Google deems 'helpful' to the searcher, that is, sites with useful content.
I've long been cynical about the algorithm's ability to 'judge' (which is difficult and subjective) as opposed to 'measure' (which is easy and objective). Expect howls of anguish and cries of 'unfair' from some quarters.
What will be interesting is the effect, if any, on the rankings of individual pages. Google used to rank pages based on their content, links and so on. Now, part of that ranking is derived from the whole site. Will a 'non-helpful' page (eg. an ecommerce product page) rank better because the rest of the site is deemed helpful (information pages) ? Who knows.
What seems clear is that, if your site is hit, there's no quick fix.