ACF allows you to add many field types; text, numbers, single / multiple-select, colours, images, taxonomy and even relationship fields. You create any number of new field types and associate your new field group to your posts and pages. These new field types then show up in the editor allowing you to add new metadata to a standard WordPress post.
For example, on this website, I have additional fields for this post type to allow me to add links to the web pages of the services I use.
ACF was bought recently by Delicious Brains, the team behind the very useful WP Migrate plugin and SpinupWp, the hosting management system that I use for all of my WP websites.