I'm only a learner, so I could be wrong. It means: (If you had done X, then maybe Y would of turned out differently...but you didn't).
examples:
네가 음식을 요리할 수 있다면 몰라도 지금 먹을 수가 없어.
"If you could cook (but you can't) then we would be eating now" / (This situation could of turned out differently if you could cook, but because you cannot, we are not eating at the moment)
다면 몰라도 = carries the nuance of 'unless'
네가 입시를 합격했다면 몰라도 지금 대학교에 다닐 수가 없어.
"Unless you pass the entrance exam, you cannot attend university" / (If you had passed the exam, you would be able to attend university, but because you haven't, you cannot go)
네가 더 예뻤다면 몰라도 결혼할 수 없어
If you had been prettier, maybe you could be married / (If you had been pretty, then maybe you could get married, but because you are not, you cannot get married)
I hope this helps.