There aren’t any specific details on what is in the update, but we do know it is delivered automatically. Several users over at MobileRead Forums have already reported being surprised by the update. Apparently the Kindle Fire is set to check for updates and then download one if available.
That’s not good news for anyone who has rooted their KF; the update removes the root and returns the tablet to its regular condition. Luckily the update doesn’t appear to make any permanent changes; there are already reports that the KF can easily be rerooted after the update is applied.
You can download the update and apply it manually if you like, but there’s really no need. Your KF will install it on its own.