A block theme is a WordPress theme with templates entirely composed of blocks so that in addition to the post content of the different post types (pages, posts, …), the block editor can also be used to edit all areas of the site: headers, footers, sidebars, etc.
A block-based theme is a theme that takes full advantage of blocks and the full site editing capabilities of WordPress 5.9. It requires a theme.json and usually includes a few templates and/or template parts. With a block theme, the user can edit all parts of their website without code
Fast to start, but takes time to master.