it's simple, Hangul is sound characters. hanja(chinese characters)is meaning characters. we can know meaning of hanja words written by Hangul by contexts, but if we want to know more accurately, we should know chinese characters.

Well, Hanja or Hanmoon we call for the chinese charater has been learnt as a subject after primary school for the next 6years for middle and high school in Korea, at least in my school days. Nowadays, the frequency of hanja used in Korea is rare and getting decreased, and they just learn as a subject like English or the other subjects of school curriculum, but hardly used out of school. However, even if we use our unique character Korean, many of words are from Chinese in meaning, and similar sound of them at some point. That's why you may find similar words in among Chinese, Japan, and Korean.

왜 한국과 일본이 한자를 사용하는지, 그리고 왜 한국과 일본이 한글이나 카나를 사용하면서 굳이 한자를 버리지 못하는지를 알려면, 한자의 역사를 알아야 합니다. 비록 중국인들이 한자를 그들의 문자로 사용하고 있지만, 한 가지 분명한 것은, 한자를 만든 고대 동이족은 바로 한국인의 조상이라는 것입니다.