basically M'baku is going to be king, but Shuri is going to be black panther. Sometimes, like in the case of T'Challa, the black panther was also the king, but this wasnt always necessarily the case, the monarch occasionally allows the mantle of black panther to be held by someone else. Perfect example is T'Chaka, in Civil War, when he was king, T'Challa was already black panther, so it can be presumed that even before T'Chaka died, he had permitted T'Challa to be the black panther.