What are Mailchimp merge fields?
The information you collect through Mailchimp or synced from Salesforce can be saved in an audience field and tied to a unique label called a merge tag. You can use merge fields for both segmentation and personalisation. Field mappings allow you to map data between Salesforce and Mailchimp, specificially to merge fields. For more information on merge fields please see - https://mailchimp.com/help/getting-started-with-merge-tags