Matt Cutts has confirmed that Googlebot can now read AJAX & JavaScript for indexing some dynamic content on webpages.

Statement from Matt Cutts about Google's ability to read AJAX and JavaScript

What does that mean?  Many websites that accept comments have been using some kind of scripting software.  Previously, Google could not read these comments.  One common one that many people have implemented is a Facebook comments script.  Google can now index these comments, so they now become part of the content of the website, where previously they were ignored.