Not to nitpick, but the couplet starts with:
ساحرُ المقلةِ معسول اللمى
جال في النفس مجال النفس
The following couplet is:
سدد السهم وسمّى ورمى
ففؤادي نهبة المفترس
In Fairouze's version it is: سدد السهم وأصمى إذ رمى -- في فؤادي نبلة المفترس (if this is where you got the words from).
These are the final two couplets where as the one you gave comes at the beginning and does not contain the word غطاء
Sorry about that I just thought you might want to know.
Anyhow, back to the question. In the couplet in question he is describing the one he loves saying that she has enchanting eyes, sweet lips, and that she roams inside him as his breath does.
Just to clarify the meaning, the verb جال here means to roam, tour, or 'go and come' - from it the noun جولة= trip. I understand مَجال here as مصدر (although when searching in dictionaries I found that the two maSdars for this verb are جَوْل وجَوَلان) and the expression is مفعول مطلق لبيان نوع الفعل = basically a way to express the kind of action mentioned in the verb, sort of like a simile although it's technically not.
On the other hand, the word مجال is actually اسم مكان, similar to the word مكان itself! It means "the place where roaming occurs". In this case the more accurate translation would be: she roams inside him where his breath does. I went for the first one instinctively, and I'm not sure if it's correct grammatically to use the second case in this way, the meaning however, works in both cases.