"You are the one." is always said in context. If a man says that to a woman (or vice versa), it means that "You are the one that I want to be with for the rest of my life." It means that the person is special in some way. It is usually used romantically, but it can also mean that the person is the best at some field.

Are you the one (the one and only person) for me? That might me a common question in this context - well it is more or less 'unique'. 'The one' is often used in cases of prophesies in myth and literature meaning 'the one predicted'. It is less 'unique' than 'singular'