WordPress Gutenberg is a new editor for WordPress that, according to WordPress “A new publishing experience for WordPress is in the works: get ready to make your words, pictures, and layout look as good on screen as they do in your imagination, without any code.”
It treats sections of the content (Headings, paragraphs, lists etc) as “blocks” allowing for individual formatting. Unlike the current editor, where you have a visual and text (html) view, under Gutenberg, there is one visual view and the text view is handled at block level.
Gutenberg will be of great assistance to those with little to no understanding of HTML who might faint at the idea of looking at the text view. While I can see the value of its use, given the proliferation of themes that already take a block approach to page construction it is hard to see how it would fit it or its impact on future theme development.
Although it is available as a plugin, it is still very much under develop. Like the future release of Gutenberg (pegged for WP V5) its impact on themes is basically wait and see.