External objects are supported in API version 32.0 and later. External objects are similar to custom objects, but external object record data is stored outside your Salesforce organization. For example, perhaps you have data that’s stored on premises in an enterprise resource planning (ERP) system. Instead of copying the data into your org, you can use external objects to access the data in real time via web service callouts.

External objects are available with Lightning Connect and Files Connect. Each external object is associated with an external data source definition in your Salesforce organization.

An external data source specifies how to access an external system. Lightning Connect uses external data sources to access data that’s stored outside your Salesforce organization. Files Connect uses external data sources to access third-party content systems. External data sources have associated external objects, which your users and the Force.com platform use to interact with the external data and content.

By accessing record data on demand, external objects always reflect the current state of the external data. You don’t have to manage a copy of that data in Salesforce, so you’re not wasting storage and resources keeping data in sync.

External objects are best used when you have a large amount of data that you can’t or don’t want to store in your Salesforce organization, and you only need to use a small amount of data at any one time.

For more info, check the below link

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

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