This is not an idiom — it is the ordinary conditional -(으)면 'if, when' landing on 마시다 'drink'. Several lines are social rules about drinking: 이러면 마셔야 돼요, said of a drinking game where missing the beat costs you a shot, and 높은 사람이 술 이렇게 주시면 마셔야 되고, 'if a senior pours for you, you drink'. Two other lines are the fixed word 아니면 'or else', where the -면 is not a real condition at all.