"내게 얼마나 큰 의미인 걸까" = 내게 얼마나 큰 의미인 것일까
... 큰 의미인 것일까 = 큰 + 의미 + 이다 + ㄴ 것 + 이다 + ㄹ까.
Compare it with: 그것은 내게 큰 의미인 것이다.
- 그것은 내게 큰 의미이다: It has[is] a great meaning for me.
- 그것은 내게 큰 의미인 것: (the fact) that it has a great meaning for me.
- 그것은 내게 큰 의미인 것이다: It is that it has a great meaning for me.
(In Korean, this structure is used a lot more than in English)
~인 것일까 is "Is it that..?", te question form of "It is that..".
이다 (is) therefore appears twice in the sentence (인 and 걸까 = 것일까)..
To reflect the structure of the original sentence, it is like:
How great a meaning it *is* to me _is_ what I wonder (*is* = 인; _is_ = 일까).
So it means "I wonder how great a meaning it has for me" or "I wonder how great it means to me".
* "내 낡은 기타을 들어" => (I) pick up my old guitar.
들어 is a conjugation of 들다 (pick up, lift).