The External API activity is a Data Management activity. It allows you to call an external REST API endpoint. The purpose of this activity is to get personalization data from a third-party system into your campaign.
Example use cases include:
This video demonstrates the use of the External API activity.
External API activity (06:48 min)
The activity is meant for fetching campaign-wide data, not for retrieving specific information for each profile as that can result in large amounts of data being transferred. If the use case requires profile-specific information, the recommendation is to use the Transfer File activity.