This powerful plugin is a great way to graphically add beautiful fields and data to every post of a certain type, extending the already-rich WordPress functionality in a great way that I personally love and use all the time.
ACF allows you to make logical rules about when certain fields are shown, which post types they are for, how they are accessed, conditional logic for given fields and a whole lot more. There are not enough good things for me to say about this plugin.
There are plenty of other ways to add fields to WordPress posts since it is such a useful functionality but this uses easy-to-create logic rules to add fields where they are needed instead of relying on code which many of the other solutions do which is limiting.
The ACF pro version also allows for the creation of custom blocks for the Gutenberg editor. I will not explain all of the nuances of block creation, but ACF is a great way to graphically achieve a lot of great things in a WordPress site without having to write much code! (although values given into ACF are accessible via code too!)