This word describes someone without a father, or someone that has been left by his father after his birth. You have to be very close to the person you call Kopele, otherwise it's very offensive! It's like the N word for people with black background. Mostly used in Bulgaria.
Ko staa we kopele? - What's up bastard?
or.. you can use it call or talk about someone:
That dirty kopele!! - same as: That dirty son of a bitch!