When a record contains a Linked Field (such as the Employer field on a Person record, the Prospective Customer field on an Opportunity record, the Primary Contact field on a Meeting record, etc), you might want the same values from fields on the linked record to appear in fields on your form. You can control whether those values appear in new custom fields on your form or in existing fields on your form.
For example, if you want the Postcode from an Organisation record to appear on an Opportunity record, you should create a custom field of type Text on the Opportunity record, then open the Linked Fields tab and choose Prospective Customer.
In the dialogue box that appears click Add mapping. Use the dropdown picklist next to Value Returned to select Postcode/Zipcode and the dropdown next to Form Field to select the name of the custom field you created, as shown in the screenshot below (click to enlarge).