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.
When importing custom objects, solutions, or person accounts, you can use external IDs to prevent duplicate records from being created as a result of the import operation. An external ID is a custom field that has the “External ID” attribute, meaning that it contains unique record identifiers from a system outside of Salesforce.