This is the property of Link object.
Primary Id Field is the optional property and specifies the name of the field in the master business component that holds the row ID values pointing to primary records in the detail business component.
Let me explain this to you, Lot many developer face problem to understand this properly
We have an Opportunity and a sales team associated with that.
We click on sales team which has MVG applet associated with that. In MVG applet we select a sales person as primary by checking the primary flag to TRUE.
Now the primary sales person, the row id of this record will be stored in master BC field, this will be the field which is mentioned in the primary Id field of link object.