This is a receptor located in the mouth of the snake which senses particles collected from the air by the snake's flickering tongue. This is how snakes can "smell" with their tongue. It is also found in other animals, including some mammals and, according to some researchers, in humans.

It's a series of nerve endings in the roof of the snakes mouth, used to collect and translate information when the snake flickers it's tongue to smell.

The snake has a stongue which is known as the jacobson's organ.Through this the snake smells the air in the environment and finds its prey in the surroundings.This organ is also present in few other animals like the Chameleon, Komodo dragon,etc.

"The vomeronasal organ (VNO), or Jacobson's organ, is an auxiliary olfactory sense organ that is found in many animals. It was discovered by Ludwig Jacobson in 1813. During embryological development, it forms from the nasal (olfactory) placode, at the anterior edge of the neural plate (cranial nerve zero). It is a chemoreceptor organ which is completely separated from the nasal cavity the majority of the time, being enclosed in a separate bony or cartilaginous capsule which opens into the base of the nasal cavity. It is a tubular crescent shape and split into two pairs, separated by the nasal septum. It is the first processing stage of the accessory olfactory system, after which chemical stimuli go to the accessory olfactory bulb, then to targets in the amygdala and hypothalamus. The vomeronasal organ is mainly used to detect pheromones, chemical messengers that carry information between individuals of the same species, hence is sometimes referred to as the "sixth sense." The VNO has two separate types of neuronal receptors, V1R and V2R, which are seven-transmembrane receptors that are coupled to G proteins. The receptors are distinct from each other and form the large family of receptors in the main olfactory system. Evidence shows that the VNO responds to nonvolatile cues which stimulate the receptor neurons. Information is then transferred to the accessory olfactory bulb as well as other centres of the brain such as the anterior part of the hypothalamus. Its presence in many animals has been widely studied and the importance of the vomeronasal system to the role of reproduction and social behavior (through influence on anterior hypothalamus) has been shown in many studies. Its presence and functionality in humans is widely controversial, though most studies agree the organ regresses during fetal development." "The functional vomeronasal system is found in many animals, including many snakes, and mammals such as mice, rats, elephants, cattle, dogs, goats, and pigs. Salamanders perform a nose tapping behavior to supposedly activate their VNO. Snakes use this organ to sense prey, sticking their tongue out to gather scents and touching it to the opening of the organ when the tongue is retracted." Source and further information: http://en.wikipedia.org/wiki/Jacobson's_organ