What is an external ID in salesforce?

An external ID in Salesforce is a unique identifier used to reference records from external systems, allowing for easier data integration and synchronization. It can be a field in a Salesforce object marked as an external ID, enabling operations like upserts (update or insert) without relying solely on Salesforce's internal record IDs. This feature is particularly useful for managing data imports and maintaining data integrity across multiple systems. External IDs can be set to be unique and are often used in conjunction with APIs for data management.