This is a common French phrase that means "Where are you from?"
The phrase you provided, "Dou viens-tu?", is a slightly informal way of asking this question. It is most commonly used in spoken French and can be considered more casual than the standard "Où viens-tu?".
Here are some other ways to say "Where are you from?" in French:
* D'où viens-tu ? (more formal)
* De quel pays viens-tu ? (literally, "From what country are you?")
* Tu es d'où ? (informal, literally, "You are from where?")