Explanation
Process Builder is an automation tool that allows you to create processes that perform actions based on criteria that you specify. You can use Process Builder to update fields on related records when a record is created or updated. To meet the requirement of updating the parent object Referral when the Referral Date field is updated on the custom object Referral Source, you need to create a process that triggers when a Referral Source record is updated, checks if the Referral Date field has changed, and updates the Referral Date field on the related Referral record. References:
https://help.salesforce.com/s/articleView?id=sf.process_overview.htm&type=5