Well i was going to offer my own interpretation until i came across Mr. Matt Cutts's version, and well his is just a tiny winy better.
In other words, when there’s newer content published on a site, it won’t get listed in Google’s index immediately.
Meet Caffeine, Google's current web indexing system. Their able to analyze the entire web continuously, enabling them to update their index with newer sites or information on current sites immediately as their found. Actually Caffeine went live on June 8th 2010 !
Make no mistake, this wasn't some minor optimization.
Caffeine takes up nearly 100 million gigabytes of storage in one database and adds new information at a rate of hundreds of thousands of gigabytes per day. You would need 625,000 of the largest iPods to store that much information; if these were stacked end-to-end they would go for more than 40 miles.
With Caffeine, the search giant has taken another step to give us more accurate, faster, and updated search results. In my opinion, I feel optimizations like this will lead the progression of Web 3.0 as it’s a clear sign that the amount of content in Web 2.0 needs a newer way of organization and management. Don’t you?
Carrie Grimes, Google blog [Online] Retrieved 21st February 2011