雪恨 is a formal separable verb (离合词) meaning 'to avenge' or 'to wreak vengeance'. The first character 雪 acts as the verb and 恨 as the object, so you can insert other elements between them — for example 雪了恨 ('avenged'), 雪过恨 ('have avenged'), or 雪不了恨 ('cannot avenge'). It is used in written or elevated speech, often referring to righting a past wrong with dramatic force.