가셔야 해요 is honorific expression of 가셔야 해 가셔야 해요 << 가(다) + 시 + 아/어/여 하(다)+ 아/어/여 + 요 가다 : to go 시 : honorific suffix indicating the subject person is elder than the speaker 아/어/여야 하다 : have to, must 아/어/여: ending suffix 요 : honorific ending suffix 가야 해 informal 가야 해요 honorific for listener (used when the listener is elder than the speaker and the subject is younger than the speaker) 가셔야 해 honorific for subject (used when the listener is younger than the speaker and the subject is elder than the speaker) 가셔야 해요 honorific for subject and listener

It is honorific (base form ~시다), which means you are directing it at, or talking about someone other than yourself. It is used to show respect and often used when talking to strangers or to/about elderly people.

When you conjugate a verb informally and then add 야 해요 to it, it means "I have to _____". For example: 먹다 in present tense informally is 먹어, and then you add 야 해요 making 먹어야 해요 Other ones are: 보다 (to see) ------> 봐야 해요 멈추다 (to stop) -------> 멈춰야 해요 In this case, we use the verb 가시다 which is the honorific version of the verb 가다 [to go] so it means 'It is raining so you have to go quickly" with the honorific verb 가시다.