What is the unique meta key option

Every field has a "unique meta key" setting that must be filled. This is automatically created for you when you create the field. The meta key determines how the field will be stored into the database of your website. If you do not need to do any custom development you probably do not need to customize this field.

It's important however that if you change the key, it has to be a unique value that no other field has. Using the same key for multiple fields will make it impossible for the user to correctly update information about his profile or about listings.

Developers however can freely customize this setting if they know what they're doing. The metakey is used to retrieve the saved value from the database and display it. 

If you are a developer, chances are you already know about this WordPress function https://codex.wordpress.org/Function_Reference/get_user_meta and https://developer.wordpress.org/reference/functions/get_post_meta/

Note: in Posterno all meta keys are prefixed with "_" (underscore).

This means that the full metakey name displayed within the custom fields editor is "job_title", when using the meta functions linked above, you'd be using "_job_title" as meta key.