What is External Object in Salesforce? How do we use this object in Salesforce ?

External objects are similar to custom objects, except that they map to data that’s stored outside your Salesforce org. Each external object relies on an external data source definition to connect with the external system’s data. Each external object definition maps to a data table on the external system.

https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_external_objects.htm