This is the long-awaited, much-discussed (at least if you follow WordPress closely) “Try Gutenberg” prompt. Gutenberg is the next-generation editing tool, which is going to replace the “classic” editor we’re all used to. As of the next release, a few months from now, it will be the default.
We’ve already installed the Classic Editor, and you’re free to make that the default on your own sites. The Gutenberg editor is a drag-and-drop editor that works by putting everything into blocks, which can be positioned and styled however you like.
You can learn more about Gutenberg on wordpress.org. Depending on how you’re used to working, you may find it redundant, confusing, awesome or some combination of the above. Because we’ve installed the classic editor, you’ll always be able to switch.
This is part of the update to version 4.9.8 of WordPress. Besides adding the prompt, there are a number of bug fixes and privacy updates that are good.