A mudra is a bodily posture or hand gesture that has some symbolism. We often see images of people meditating with the palm open upward, resting on the knee and the index finger and thumb forming a circle. This is a mudra. Different religions or traditions of yoga use different mudras, and they may all give different meanings to these symbolic gestures. Some of the more common hand mudras as outlined in Transitions to a Heart Centered World by Guru Rattana are below:
GUYAN MUDRA: Also known as Chin mudra, the tip of the thumb touches the tip of the index finger, which stimulates knowledge and ability. The index finger is symbolized by Jupiter, and the thumb represents the ego. Guyan Mudra imparts receptivity & calm.
The Shambhala Dictionary of Bhuddism and Zen offers this explanation for this hand symbol:
Dhyani Mudra: In this mudra, the back of the right hand rests on the palm of the other in such a way that the tips of the thumbs lightly touch one another. The hands rest in the lap. The right hand, resting on top, symbolizes the state of enlightenment; the other hand, resting below, the world of appearance. This gesture expresses overcoming the world of appearance through enlightenment, as well as the enlightened state of mind for which samsara and nirvana are one.