写信 is a separable verb (离合词) meaning 'to write a letter'. The two characters 写 and 信 can be split apart when you need to insert a measure word, time expression, or complement between them — for example, 写一封信 ('write one letter') or 写了三封信 ('wrote three letters'). You also place the recipient before the whole phrase using 给: 给他写信 ('write him a letter').