What does a thin scarlet string worn as a bracelet or band on the left wrist mean?

Wearing a thin scarlet or crimson string as a type of talisman is a Jewish folk custom as a way to ward off misfortune brought about by the "evil eye". The tradition is popularly thought to be associated with Kabbalah and religious forms of Judaism.

The red string itself is usually made from thin scarlet wool thread. It is worn as a bracelet or band on the left wrist of the wearer, knotted seven times. The person has to knot it 7 times while saying the kabbalah bracelet prayer.

Red strings around the wrist are common in many folk beliefs; for example the kalava is a Hindu version. There is no written mention in the Torah, Halacha, or Kabbala about tying a red string around the wrist. It seems to be a custom that has been around since at least the early 1900s.