I am getting an error in Microsoft Access. What does this mean?

Microsoft Access Microsoft Access can't create this relationship and enforce referential integrity. Data in the table Branchesviolates referential integrity rules. For example, there may be records relating to an employee in the related table, but no record for the employee in the primary table. Edit the data so that records in the primary table exist for all related records. If you want to create the relationship without following the rules of referential integrity, clear the Enforce Referential Integrity check box. OK