Have you heard about the WordPress plugin that seems to do nothing, but is installed on over 100,000 sites?
However, you would never realize how important CMB2 just by installing it from WordPress.org. When you install CMB2, it is invisible - your site will not change in any way.
CMB2 is short for "Custom Metaboxes", but that name probably doesn't help if you're new to WordPress development.
All-in-all, I think it's time for some explanations ...