Amazon announced an update (5.3.4) to the Paperwhite software. If you have a Paperwhite you can make sure it is connected to wifi and it will likely update itself eventually. If you want to force it to update, you can manually download and update the Kindle.
From what I can tell, no one has figured out what this update is for. It is listed as a ‘general improvements’ update, but all of the coverage I have seen says that no one can tell what it does. Likely it is fixing some small bugs and/or improving the background code.