A mudra is a Seal or gesture. Most mudras are described in the Gheranda Samhita and the Hatha Yoga Pradipika. There are around 50 mudras. It usually involve the hands and fingers, but some mudras involve tounge or perineum.
The Chin Mudra opens your spiritual body, the Self. You sit cross legged, thumbs and forfinger touching to make a circle. The rest of the palm faces the sky. Place both hands on your knees and observe the space between your thumb and forfinger.