What is the field 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 it's information.
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
This function is used to retrieve the value of a user custom field from the database through the user id number and the metakey name.
In WPUM every custom field metakey has the prefix "wpum_".

This means that the full metakey name is wpum_field_11. If you change that to anything else like "job_title", the metakey will be wpum_job_title.

Here you can find an example of the get_user_meta function retrieving a WPUM field value.

get_user_meta( $user_id, 'wpum_field_11', true );