Dropping subject in imperative

Just to add some examples explicitly to Maarten's response:

Vous parlez français. -- (Statement) You speak French.
Parlez-vous français ? -- (Question) Do you speak French?
Parlez français! -- (Order) Speak French!

I have seen some people getting confused by finding an indirect personal pronoun in a command and mistaking it for the subject. Here's a minimalistic example that can be confusing to parse:

Vous vous parlez. -- You talk to yourself. (The first vous is the subject, the second the indirect object "to yourself").
Vous vous-parlez? -- Are you talking to yourself?
Parlez vous! -- Speak to yourself!

The identification of what's what becomes clearer if you substitute a name for the person to speak to:

Vous parlez à Jeanne. -- You are talking to Jeanne.
Parlez-vous à Jeanne? -- Are you talking to Jeanne?
Parlez à Jeanne! -- Talk to Jeanne!

Note, however, that the word order between using a personal pronoun and a name changes. So, as a last group of examples, let's replace "Jeanne" by her indirect pronoun lui.

Vous lui parlez. -- You are talking to her.
Lui parlez-vous? -- Are you talking to her?
Parlez lui! -- Talk to her!

As noted in the lesson, the subject pronoun is dropped in the imperative (as in English). "Parlez-vous .... would be a question/request, not an imperative. If needed to specifically identify the intended subject of the imperative statement (one from amongst a group for instance) it would be in the form (as in English) "Vous, parlez ...." or "Tu, parle . . . ", or by name "Pierre, parle(z) ...."