This is powerful thread worn by the Hindus. In the case of small children, it is usually tied around their waist and adults tie it in their left wrists or armlet. Some also tie a special root along with it and wear it as a necklace. People who practice black magic/Tantric vidhya may wear in their right foot as well.
Significance: It is said to keep You and your children away from evil eye (buri nazar).
It also keeps away people from evil spirit or unwanted tantra mantra done.