Advanced Custom Fields is a plugin that creates custom fields on post type screens. Those fields can (currently) be static fields below the content or reside in meta boxes. The sites homepage got a dedicated documentation page and a dedicated support forum as well as changelog and/or todo list.
Culprit of this plugin (currently) is that it doubles each custom field entry: One entry for the custom field ID and another one to hold the value.
If you're cross posting the same question, please play nice and add a link on both ends!
Keep in mind that there're alternatives like Pods.