phoeislost knows! Only if your name is Amelia Pond, the girl who waited.
Wall is a barrier, something is stopping you. The crack means something breaking through, the wall is weakening.
That or there is a tear in time and space. But never fear, the Doctor will be there soon.
If I were to guess,I would say it means that you are breaking through some mental or emotional struggle or maybe it could mean some source of information is breaking through your stubborn ways!:)But,who knows?