From Amazon

“As you can see, we have added many more options to the custom post type with this code. It will add more features like support for revisions, featured images, custom fields, and more.

We have also associated this custom post type with a custom taxonomy called ‘genres’.

You may also notice the part where we have set the hierarchical value to be false. If you would like your custom post type to behave like Pages, then you can set this value to true.

Another thing to notice is the repeated usage of the twentytwentyone string, this is called ‘text domain’. If your theme is translation ready and you want your custom post types to be translated, then you will need to mention the text domain used by your theme.

You can find your theme’s text domain inside style.css file in your theme directory. The text domain will be mentioned in the header of the file.”

-Amazon