Are you Getting Big traffic change for top URL Messages in your Google Webmaster Message BOX. If yes then you need to Search the reason with the help of your Message ID , Provided along with that Message. Like for Example Message ID: 593f1ceb2d67.
You will get this message when you started getting a sudden traffic in your website. Google warned you by sending this message to keep maintain quality of your website and don’t violate google webmaster guideline.
John Mueller Webmaster Trends Analyst at Google Describe this issue as
Generally speaking, there are a number of reasons why this kind of change might happen. If you’re not aware of anything technical (such as crawl errors), or any changes that you’ve made (URL removals, changes in canonicalization, etc), then it’s possible that our algorithms have re-evaluated how they consider your site’s content and/or links to that content. For example, it appears that your site is primarily focused on affiliate sales. That’s generally fine, provided that it also provides significant unique, compelling, and high-quality content, so that users love to recommend *your* website over others that provide similar information. We have more information on this topic in our help center at:
I hope all above helps , If you are not using any illegal way to get traffic or if you are not violating any webmaster guidelines then you don’t need to worry about this message.You can also also find your answer in Google Webmaster Help forum.