退款 is a separable verb (离合词) meaning 'to refund' or 'to reimburse someone for money paid'. The verb 退 ('to return') can be separated from the object 款 ('money') when inserting a measure word or aspect marker between them, like 退了款 ('returned the money') or 退过一次款 ('refunded once'). It is also commonly used as a noun meaning 'a refund' (the returned money), but the core verb-object structure focuses on the action.