"Books are meaningless."
Translation:책은 의미가 없습니다.
It's to do with the type of sentence. Another way to phrase it, I believe, is "Books have no meaning". According to the Course Notes, an "owner" of something takes "은" (if the noun ends in a consonant) or "는" (if the noun ends in a vowel). So in this case, "첵" (cheg - book) is the "owner" of "의미" (uimi - meaning). "첵" ends in a consonant, so it takes "은". Hence, "첵은" at the start. Then as the thing owned, "의미" has to take "가" or "이", and as it ends in a vowel, it takes "가", hence "의미가" in the second part, and then you add the negative verb form at the end.
I hope I've helped and not added any more confusion, and I'm certainly open to correction as I'm still trying to work all this out myself.
Its a matter of how you want to construct the sentence to answer an implicit question or place emphasis on different words. At the fundamental level, topic and subject markers are just used to mark topics of the conversation or distinguish actors (subjects). Everything beyond this purpose is contextual to the surrounding conversation.
- 의미 = "meaning", noun
- 의미가 = 의미 tagged with the subject marker 가
The sentence formation: "X는 Y가 없습니다" translates to "X does not have Y".
Subject markers are a nuaced part of Korean grammar, so be sure to read up some more about them. There are plenty of videos and articles outside of Duolingo that do a good job of explaining them.