That means, most importantly, that you won't be able to download new books. The emergency update is for the original Kindle Paperwhite (2012) and every Kindle before it. If you do not update your Kindle before that date, Amazon says you'll receive this warning when you try to access internet services:
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.
All Kindles download and install the latest updates automatically if they're connected to Wi-Fi while asleep. If your Kindle is sitting somewhere in your house unplugged, however, it likely isn't updated with the latest version, and you should turn it on and make it pull the new update by clicking "Sync and Check for Items" in the menu.
Fortunately, you won't be entirely out of luck if you forget to update your device (hello readers who search for this article months from now). You'll just have to do it manually by downloading the update file from Amazon's website and transferring it to your Kindle via a USB cable. For more detailed instructions, select your device on this help page. Of course, if it's not March 22nd, 2016 yet, just connect to Wi-Fi and do it the easy way right now. A full list of affected Kindle devices follow.
- Kindle 1st Generation (2007)
- Kindle 2nd Generation (2009)
- Kindle DX 2nd Generation (2009)
- Kindle Keyboard 3rd Generation (2010)
- Kindle 4th Generation (2011)
- Kindle 5th Generation (2012)
- Kindle Touch 4th Generation (2011)
- Kindle Paperwhite 5th Generation (2012)