Mudras, also known as "seals" or "locks," are gestures of the hands. When you see yogis meditating with their hands resting on their knees, thumb and forefinger touching, they are performing a mudra. But why are they doing this?
Because it's believed that by placing the hands in these various formations, yogis are moving energy into the places in the body that typical yoga postures can't reach. Cool, huh?