You can say either "체계적으로 배운다니 그게 무슨 뜻이에요?" or "체계적으로 배운다는게 무슨 뜻이에요?" (What do you mean by... ☞ ..ㄴ다니 그게 무슨 뜻이에요?) \^o^/

Hi, Margo. I think you do speak Korean well. 체계적으로 한국어를 배운다면 - I assume that you know the meaning of this sentence. The case could match perfectly with studying English. When you study English, you need to study how to phonics, structure, grammar, vocabulary and so on. I am sure you had some systematical curriculum or syllabus to follow. You would need similar strategy to study other language such as Korean and Chinese. So I can say that your Korean friend meant that you need to study Korean language with reliable strategy.