The Unity Editor uses 32-digit hexadecimal strings called Globally Unique Identifiers (GUIDs) to identify Assets within a project.
Each Asset is assigned a GUID which is stored in that Asset’s .meta file. A Unity GUID looks something like this de9a32f15f2628044842629a83d3d974
Unity uses GUIDs to track references between Assets. If an Asset’s GUID changes, all references to that GUID will become broken.
You can read more about how Unity uses GUIDs to track references here. To learn how to avoid broken references head over to Avoiding Broken References in Unity Projects