Relating imported records with External IDs
Key Takeaway
In the Data Import Wizard, an external ID matches file rows to existing records and links child rows to their parents. An auto-number external ID can't match existing records, but its values can still look up a parent record that contains the same external ID.
An external ID is a field that stores a record's key from a system outside Salesforce. When you import with the Data Import Wizard, you can match rows in your file to existing records by that key, so the import doesn't create duplicates. You can also link each child row to its lookup or master-detail parent through the parent's external ID. Your file can then use the outside system's keys instead of Salesforce IDs.
What an external ID is
An external ID field can be of type text, number, email, or auto-number.
Your billing system gives each customer a number such as C-20417. You add a custom text field named Billing Customer Number to Account and give it the "External ID" attribute. Each account in Salesforce now carries the same key that the billing system uses.