It is customary to tie a red thread - commonly called a mauli or kalava - on the wrist at the beginning of a religious ceremony. The thread is tied on the right wrist of men and the left wrist of women. A person often puts on the red thread on one wrist when doing a ceremony, ritual or puja, or for certain blessings. The thread helps preserve or imbibe those blessings when it is tied around one wrist during the ceremony.
Mauli or raksha sutra consist two colours ie. red & yellow separetaly in various parts of mauli. Yellow colours represents the spritual energy, while red has the properties to stimulate the spiritual energy. It protects the wearer from negative energy. It is also offered to deities during worship as Vastra (cloth offering).
The literal meaning of mauli is 'above all'. Here the reference is to the head that stands high. With the moon perched on top of Shiva's head, he is referred to as Chandramauli. The practice of tying the thread dates back to the time when Vamana Bhagwan tied this holy thread on the wrist of the progressive King Bali to grant him immortality.
The thread can also be called a raksha or rakhi, and is put on the brother by the sister to show the sister familial love. The knots in the thread are said to hold the love of the sister when she tied the knot. Thus, the brother wears the rakhi as a sign of his sister love and wishes for protection. In other arrangements, the knots tied in the rakhi as the red strings are given to guests or tied around their wrists before a special ceremony or reception is also considered to hold the goodwill of those who offer it.