It is worn by many people regardless of their religious beliefs or country of origin.
It is believed that a Red String bracelet worn on the LEFT wrist protects the wearer from NEGATIVE energy to enter his body, this powerful negative energy is sometimes associated with The Evil Eye, or refers to the unfriendly stare and unkind glances we sometimes get from people around us.
"Hamsa" sometimes also called Fatima's Hand is the most popular cameo for good luck and well being around the world, the central bead looks like a blue eye, and it is part of the Hamsa cameo, against the negative energy, or Evil Eye.
Around the world in ancient civilizations these symbols have had various meanings, but in the end they all stand for the same basic purpose, to rid of negative energy, protect the wearer and invite good energy and success in his/her life.
In Eastern societies it is believed that wearing the Red String will help one find his/her true soulmate. It is sometimes tied around the wrist, finger, or ankle in Japan.
In Hinduism, a red string bracelet is used in many of their ceremonies, and it is supposed to invoke the divine blessings, fortune, well-being, and prosperity.
For those who follow "Kabbalah" a red string bracelet is worn to ward off all evils and negative energy.
Kabbalists are of the belief that if someone is jealous of you, they look at you with the "evil eye" and thus, negative energy enters your life, which can harm you in many ways.
They wear this bracelet, at all times, as according to their religion, it will eliminate all kinds of negativity from your life.
Buddhist Monks are also known to wear red strings around their wrists, and sometimes their necks. Red is a powerful color with great significance in this ancient religion. It is believed to bring good energy as well.
Although, a red string is known to bestow good fortune on a person, this is just a belief, a religious myth, which has been carried on for generations. There is nothing concrete to prove, whether it actually does what the religious beliefs say. Still, as religion is something to be believed in and not dissected, anyone can wear a red string bracelet and see for himself, whether it really works wonders or not!
If you are curious to know more here is an interesting article: http://www.buzzle.com/articles/red-string-bracelet-meaning.html