发信 is a separable verb (离合词) meaning 'to send or post a letter'. When you add a measure word, time expression, or aspect marker, the two characters split: the verb 发 takes the grammar, and the object 信 follows after. For example, 发了一封信 ('sent a letter') or 发过几次信 ('sent letters a few times'). The word can also refer to sending a message in modern contexts (email, text), but the core meaning is sending a physical letter.