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)
Please note that the activity is meant for fetching of campaign-wide data (latest set of offers, latest scores etc.), not for retrieving specific information for each profile as that can result in large amounts of data being transferred. If the use case requires this, the recommendation is to use the Transfer File activity.