Nowadays there is an API Integration in almost every org. The main purpose of integration with another service is to avoid reinventing the wheel. However, the development effort that is required to integrate with services is a complex and time-consuming venture. It reduces speed to market, but it also saps developer energy that is better spent in the front end, building the features that will really differentiate their app. With Lightning External Services, Salesforce makes this a lot easier and admin friendly.
With external services you can connect to any service that you want to, invoke methods based on the external source via a flow all with the help of an easy-to-use wizard. Declarative tools are used to import API definitions right into Salesforce. Swagger or Interagent-based API definitions can be used to define an external service. Once the definitions have been imported, you can create lightning flows which will invoke actions generated from the API definition schema. Below is a depiction of how external services works.