Most Kindle updates focus on nice-to-have improvements, but this is one you won't want to ignore. Amazon is warning owners of pre-2013 Kindles (that is, the original Kindle Paperwhite and earlier) that they need to update to recent software before March 22nd if they want to stay online. If you're rocking one of the older e-readers and don't heed the advice, you'll lose access to the Kindle Store, your books in the cloud, and anything else that depends on Kindle services -- basically, some of the reasons you bought a Kindle in the first place.
You're not completely out of luck if you miss the deadline, as you can still recover if you're willing to manually update with a USB cable. Suffice it to say that this isn't the best way to remain current -- you're better off going wireless if you can. However you upgrade, the alert is a blunt reminder that cloud-connected devices won't work forever unless they have the long-term support to match.