In WordPress 5.0, the Gutenberg editor will replace the longstanding TinyMCE editor with a content management experience closer to a page builder, allowing users to add blocks to posts and set which blocks can be added to custom post types.

This will be the biggest change to the WordPress user interface in a long time.  Here are a few articles to learn more:

https://wordpress.org/gutenberg/

https://go.tinymce.com/blog/gutenberg-myth-busting/

WordPress’ New Gutenberg Editor Now Available as a Plugin for Testing