External Services is a feature within Salesforce that leverages declarative tools to connect to an external endpoint, such as a credit service provider, and bring its logic into Salesforce.

Go to External Services from force.com setup to set it up. It’s easy to set up with point and clicks.

Use External Services to connect to a service of your choice, invoke methods based on the external source via a flow, and import data from the service into Salesforce—all with the help of an easy-to-use wizard.

Use External Services to connect your Salesforce org to a service of your choice. Invoke methods based on the external source via a flow. Import data from the service into Salesforce. You can do all this with the help of an easy-to-use wizard.

Trailhead link – https://trailhead.salesforce.com/en/modules/external-services

Apex Actions in Flows: