The word שום seems to have meaning other than I assumed (a Garlic). Can someone please explain it for me??
It is used in the sense of "no" to negate nouns, as in "I have no books" - אין לי שום ספרים. It implies emphasis, I have no books at all, whatsoever. In the fixed expression שום דבר it means "nothing".
In anticipation of the next question... ;-) originally it means something like "anything", so technically it isn't a double negative, but it is never used in this sense anymore, so it does feel like a double negative.