"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).

for the second question, i need to know the context. 듣다(listen) can be transformed as 들어. 들다(pick up) also can be transformed as 들어. but i assume it means "pick up" in the sentence.

의미이다 -> 의미인 것이다 -> 의미인 것일까 -> 의미인 걸까 이다 is 조사 that makes ends of sentences meaning "be". 것 is difficult to explain in english. usually it means "thing" ex) 먹는 것 things to eat. 살 것 things to buy. but when it is used "bla blaㄴ/는 것이다" at the end of a sentence, it means "it is that bla bla". Actually it has no practical meaning, but just emphasizes the fact. 것일까 means "is it that bla bla?", making a questioning sentence. 걸까 is abbreviation of 것일까. "wonder" ~ ㄴ/는 걸까? is usually used to express the wondering mind for something. hope this is help for you.