The ‘External ID’ field may be used to reference an ID from another, external system. External IDs are searchable in Salesforce and you can also use the Upsert API call with the External ID to refer to records when making changes via Data Loader.
External IDs are commonly used to store unique record identifiers from external systems and allow for routinely loading data into Salesforce without having to prepare your import file with existing or related Salesforce record IDs each time.