Hey, basically it is a superstition thing!, the purpose of tying black thread around your ankle, upper arm, etc, is to protect you from "evil", you may have heard of this "evil" as "nazar" in Punjabi/Hindi/Urdu, it is supposed to protect you from evil things such as black magic (kaala jaddu), nazar also occours when someone compliments someone (usually a new born baby) too much, you may or may not of heard of when someone is possessed, you tie their hair in a knot (like the black thread), and something happens to the thing possessing them inside, to be honest I don't know, but the hair knot thing is related to the black thread tradition, the black thread is common in Asia and Africa, I noticed it usually in Punjabi's but Africans and Asians do it as well.
