Mudra is defined as a hand position.  There are specific different types of mudra in Kundalini Yoga that are used in order to help facilitate certain types of energetic flows.  The human body is one large circuit of energy.

The energy that flows throughout our bodies follows along paths called nadis.  Certain points along these, or meridians, are conducive for the cultivation of specific types of energy. 

The study and use of Mudra utilizes the meridians to ressonate with a specific type of energy that attunes the system to the desired affect.  For example, a common Mudra, is Gian Mudra which focuses on channeling Wisdom.  The connection of the pointer finger with the thumb completes a circuit, so to speak, that is the wisdom connection.

All definitions here are based on my understanding, learning and reading of Kundalini and other Yoga related texts, classes and websites.

I invite the Kundalini Yoga experts in the community to ellaborate on what I have presented here, and thus further the learning experience for visitors.