Basically, this means that they try to notice the changes "that could be most interesting for you", and send the website owner a message when they spot something.
You can view these messages in the Webmaster tools site, or by signing up inside Wesmaster Tools to have alerts forwarded to your email.
The newest(*) alert they added is for Search Queries. ref: http://googlewebmastercentral.blogspot.ie/2012/08/search-queries-alerts-in-webmaster-tools.html
This features shows "impressions and clicks for your top pages over time."
They say
"For most sites, these numbers follow regular patterns, so when sudden spikes or drops occur, it can make sense to look into what caused them. Some changes are due to differing demand for your content, other times they may be due to technical issues that need to be resolved, such as broken redirects. For example, a steady stream of clicks which suddenly drops to zero is probably worth investigating."
They're still working on what levels of change to report: it should be interesting to see how they cope with the wide variety of seasonal patterns. For example, I have one site that has weekly content: each week's post builds interest in the preceeding three weeks, then has strong traffic until a certain day and then drops to pretty close to zero. It will get popular again, but not until almost three years have passed - simply due to the niche that this blog is in. Now, I don't want to be alerted to this, because it's pattern I know very well! But I wouldl be interested if they could tell me that the expected "strong growth in the last week" isn't happening.
(*) I originally wrote this in late August, but somehow forgot to publish it. Found it again just now, and decided to share it, even though it's not as fresh as it was then. Hopfully it will still help someone.
