These two methods of the GlideElement class exist to retrieve information about reference fields and the data that they reference.

Neither method accepts an argument (they don't need to, as they're called from the GlideElement containing the reference information).

The getReferenceTable() method returns the name of the table being referenced by the given reference element. The getRefRecord() method on the other hand, returns a GlideRecord object that's pre-populated with the record, the sys_id for which is the value of the field. In other words, the record referenced in that reference field is returned.