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.

Yes, the "you" is always hidden but implied. For me, I say the person's name so that the person knows I'm talking to him/her. It's not exactly emphasis but it's to point out to whom the imperative is being directed. Also (for me), if I just say "Stop screaming", it is almost like I have all the power and/or I am more important. I am so important that I don't have to say a person's name, I just give the orders and they are obeyed. I usually do this with my nephew to show him that I am in charge.

It is true that imperatives are second person. In your sentence Peter is being used as a vocative, showing who is being addressed