What is External Ids?
An External Id is a custom field that has the “External Id” attribute, meaning that this filed is a primary key and it contains unique record identifiers from a system outside of Salesforce. Salesforce allows up to 3 fields as External Id and these fields must be text, number or email field types.
What is the use of External Ids?
Basically we use “External Id” to match on for updates. When we perform upsert or update, we can match on this “External Id” field. External Id is a manual record Id will be used to integrate with external system.

What is the advantage of External Ids?

• The import wizard will detect existing records in Salesforce that have the same External Id.
• When using the upsert command during data loading, we can reference the External Id field instead of the Salesforce Id.
• When we load data from external systems, We can use External Ids to prevent duplicate records from being created as a result of the import operation.
• Fields marked as External Ids are searchable in the sidebar search.