Mudras are special positions of the body. Mostly the hands and fingers are held in some special position. These mudras have very special influence on the energies of the body and our chakras.
Some well-known mudras are the ones performed while meditating. One sits in lotus position and puts one's hands on the knees, the tips of the thumb and index finger joining. Mudras have been practiced across the world and in different religions. For example the Christian crossing of the fingers for prayer is also a mudra.
The indian greeting gesture with folded hands puts one into a good mood by balancing energies.
Mudras have been in use in the East for thousands of years. Lot of research work was done by great saints on balancing the chakras. Mudras were discovered to balance the chakras and to remove blockades in important energy centers of body. They're also used for physical ailments apart from spiritual reasons.
There are various types of mudras. Gyan Mudra is described here;
It means wisdom posture. It is used to receive cosmic energy.
The tips of the thumb and index finger touch, other fingers are straight but relaxed. Hold the finger positions with both hands for couple of minutes.
It stimulates the Root chakra.
It improves concentration and brings a feeling of serenity. It helps meditation.