Can anyone tell me what is subscription in GraphQL and why we need that?

I am working with graphql and using apollo for implementing it . Everything was fine till apollo-client . but then I came in a question like if I want a user Authentication then I cant have a whole valitation and authenticating method inside a single resolve method. I came to apollo-server and then it got worse. My question

• What is subscription in graphql?

• Why we need this?

• WIll subscription solve my problem of the login system ?

• if not! how will I solve it?