It's a polite way to ask someone about their language abilities, particularly if you're unsure what language they speak.
Here's a breakdown of the phrase:
* Comment: How
* Parlez: You speak (formal "vous" form)
* vous: You
Example: "Comment parlez-vous?" "How do you speak?"
Note: In informal situations, you could use the familiar "tu" form instead of "vous": "Comment parles-tu?"