What does a red line on the side of a field indicate?

A red line on the side of a field indicates that this field is required in Mailchimp. If the field is required, you must ensure it’s populated with data so the record can be updated or saved. To verify the merge field in Mailchimp, click on the dropdown next to the edit button at the top right of the field mappings screen. This will open Mailchimp and take you to the audience merge fields so you can edit them directly.  Any changes made will be automatically reflected in Salesforce.