One of the most common (if not the most common) of all hand gestures in yoga is the Seal of Consciousness a.k.a. Chin/Jnana Mudra. Whereas chin translates to consciousness, mudra meaning seal or gesture and jnana is known as wisdom.
The hand gesture is symbolic when connecting the thumb and index finger of either or both hands. The index finger represents the individual consciousness and the thumb representing universal consciousness to which the word yoga translates as the unification of both cosmically.
The other three fingers also represent a gesture towards purification from impurities of ego (middle) and the dissolving of karma (ring) and illusion (little).