What are invocable actions

Salesforce Invocable Actions are a powerful feature within the Salesforce platform that allow users to create custom logic and processes that can be called (or invoked) from various automation tools such as Flows or Apex.

This action allows you to add an email address to an audience in Mailchimp. Optionally you can choose to add Mailchimp tags. Below is an interactive video on how to add the action.