Caffeine is Google's 21st Century answer to an indexing system that was slow and stale. It provides 50 percent fresher results for web searches than their last indexing system. 

We've built Caffeine with the future in mind. Not only is it fresher, it's a robust foundation that makes it possible for us to build an even faster and comprehensive search engine that scales with the growth of information online.
-Carrie Grimes, Software Engineer for Google

Google's old Indexing system would update it's index by re-scanning the entire web and updating their database in one big hit, which meant there was a significant delay for any website updates to come through.

With Caffeine, Google is still constantly scanning the entire web, but it updates its index in smaller portions more often. This means that when searching in Google you are finding much fresher content sooner.

Still not making sense?

Just imagine if the painters of the Harbour bridge left covers over the newly painted parts of the bridge and didn't remove the covers until the entire bridge was re-painted, It would be a long while before we got to see a fresh coat of paint. Instead the painters reveal each freshly painted section as soon as a section is finished.

So how does that affect my website?

Caffeine means that Google users will be able to find your newly created, updated and scheduled webpages  much quicker, increasing the traffic to your site and the effectiveness of your analytics.

Now go grab yourself a coffee, write some new content and see how long it takes for Google to index it.