As the name suggests... whenever you goes through an "interesting" article/s on sites and thinking about sending that link using minimal of clicks directly to your friends so they wouldn't miss it, then that's what the "Send" button is all about.
|Is this the new "version" of e-mailing?...|
So as you can see with the above picture, not just sending to your friends (yes you can send it more than a single person... great now you can easily SPAM your poor friends :P) but you can add your own few "opinions" to the message as well. Most importantly, you can send these links not just for your friends on Facebook, but the "Send" feature also let you send the links directly to e-mails as well.
If you're interested in adding this feature to your web site, then please visit this official page for more.