Lauren is almost right: 'you' is always the subject of the second person imperative, and it is usually omitted/implied, but not always: it can be used to add emphasis, most commonly with a negative imperative, for example "Don't you dare!", but it is also used' in positive imperatives such as "You be careful, now".
Bramwell is right that the recipient of an order (second person imperative) such as Peter is vocative, not nominative, i.e. not the subject. This is usually written with a comma to indicate that it is unusual, and spoken with a pause: "Peter, be careful", or "You, stop right there!". The vocative and 'you' can even be used together: "Dad, you be sure to lock the door tonight!".
All the above applies to the second person imperative, which is the commonest. There are also first and third person imperatives, these have no 'you' at all.