This is a really core concept when it comes to integration.
Most information systems have unique identifiers (IDs) which are used to uniquely number each record in their system. These IDs are typically unique and internal to each system.
When we want to integrate to systems we need to align and map these IDs from one system to another.
If we are lucky the systems we are integrating have some way of storing an external ID from another system with each record. This makes integration easier.
If we are unlucky and they do not, don’t give up. Other solutions exist.