This is pretty much your final warning: If you have a Kindle, you need to update it before March 22 or it's going to lose Internet connectivity.
That, just to be clear, would be very bad, because without the update you'll no longer be able to access the Kindle Store or sync your device with the cloud, not to mention any other Kindle services you might be using. According to Amazon, the update is required to ensure the Kindle remains compliant with continuously evolving industry web standards.
You'll know if your Kindle didn't get updated in time because you'll see the following message on your device:
Your Kindle is unable to connect at this time. Please make sure you are within wireless range and try again. If the problem persists, please restart your Kindle from the Menu in Settings and try again.
If you've been using your Kindle regularly then it's most likely going to be fine. Kindles will update automatically via Wi-Fi, but if the device has been turned off or out of battery for a while, charge it up and make sure you Sync and Check for Items.