Mudra literally means "gesture". Mudra expresses and channelizes cosmic energy within the mind and body. The body as we know comprises of 5 elements- Earth, Water, Fire, Air and Space. When these 5 elements are not in balance we experience numerous physical and psychological ailments.
The energy from the body radiates out mainly through the tip of the nose, lips, finger tips and toes. When the thumb gets in contact with the other fingers, energy is directed back to vitalize various parts of our body. This is known as circuit bypass.
In our Puranas, our Gods and Goddesses are depicted in different Mudras. Mudras are the expression of our inner feeling. Mudras that are part of Yogic postures are called Yogic Mudra while, Mudras done using hands are called Hasta Mudra. The five fingers of a human hand represents 5 elements (tatvas)
Out of these, fire element is of vital importance because when fire element goes weak, various ailments come to the body. Death sets in when the fire element dies. Joining the fire element (thumb) with other elements (other fingers), vitalizes them also.