If you don’t download your Kindle books before February 26, you’ll be able to read them on Kindle devices and apps, but not on other eReaders or non-Amazon HW/SW.
You bought those books, but Amazon is about to relinquish your ownership of personal copies. Over the next seven days, take the opportunity to visit the Amazon website, go to Manage Your Content > Download by USB and download all of your Kindle books one by one.
