According to the Official Gmail Blog, the update will improve autocomplete predictions by using the content found specifically in your inbox. This means that conversations stored personally on your account can be used as autocomplete suggestions when searching through your e-mails.
“Now when you type something into the Gmail search box, the autocomplete predictions will be tailored to the content in your email, so you can save time and get the information you want faster than ever before.” said Isaac Elias, Gmail Software Engineer. “For example, you might now get lax reservation or lax united as predictions after typing "lax" if you have received an email with a flight confirmation for your trip to Los Angeles in your inbox recently.”
Google has announced that the feature will be rolling out in English over the next few days, along with more languages in the next few months. We can also expect to see support for this feature on Google Apps sometime in the future.