What does 맘이 (mam-i) mean?

See a translation

마음(맘) means usually emotion or mind.
For example,
"그 남자(That guy) 마음(mind) 속에 무엇(what)이 들어있는지 도대체 읽을 수가 없어.(=It's totally impossible for me to read that guy's mind.)"
맘 is contraction of 마음. Though 맘 and 마음 are interchangeable, 마음 is more preferred than 맘 in writing.
The literal meaning of 심장 is heart, the organ placed in the left chest and pump the blood by regular beating. However, when 심장 is used in lyrics, a poem, or an emotion, it has figurative meaning.
For example "어제(yesterday) 그(him)와 헤어지고 나니 심장(heart)이 너무아파.
(=My heart hurts so much(I'm feeling blue so much) after breaking up with him(그) yesterday.)"
In lyrics such as
"나는 심장이 없어
= I don't have a heart(I can feel nothing now because of too much sorrow)"

In short, 심장, 마음(맘) are different in the literal meaning, whereas 심장 and 마음(맘) can mean the same thing in figurative expression.