What are Mailchimp merge tags?

Custom information about your Mailchimp audience members can be saved in an audience field. Those fields are tied to a unique label called a merge tag. You can use merge tags for both segmentation and email personalisation in Mailchimp. For more information on merge fields please see - https://mailchimp.com/help/getting-started-with-merge-tags

In Salesforce, Field mappings allow you to map data between your fields and Mailchimp audience fields.