When a girl says that you are good friends, it means that she considers you only as a friend and that there is nothing serious between the two of you. Girls often say this when they don't want to give the impression that they are dating this person.
It means she and the other person are good friends. If she talking about you and you don't have anything going on with her already, it's most likely she's not attracted to you. If she's talking about someone else and everything else appears like they're a couple, it could be code for we're doing stuff but we don't want anyone else to know so we're going tell everyone we're just good friends.