Let's break it down:
* es: is (verb "être")
* un copain: a male friend
* une copine: a female friend
* du frère de: of [someone's] brother
So, the full phrase is a question asking about the relationship between someone and the brother of another person.
Here are some examples of how this could be used in a sentence:
* "Est-ce que tu es un copain ou une copine du frère de Marie ?" - "Are you a friend of Marie's brother?"
* "Elle est une copine du frère de mon meilleur ami." - "She is a friend of my best friend's brother."