What is External lookup in S child standard,alesforce?

Hi Piyush,

An external lookup relationship links a child standard, custom, or external object to a parent external object.

External objects are similar to custom objects, except that they map to data located outside Salesforce. Each external object relies on an external data source definition to connect with the data outside Salesforce. Each external object definition maps to a table that contains the data, and the object fields map to accessible table columns. The data in the external table can be searched and referenced in Salesforce using custom tabs and federated search.