what is meta-sympathetic nervous system?
As @mgkrebbs says, metasympathetic nervous system is still a matter of debate and is not fully accepted in the scientific world due to lack of research and evidence.

Metasympathetic nervous system, just like sympathetic and parasympathetic nervous system, is a part of Autonomic Nervous System.


  On the basis of a number of structural and functional indices, application of pharmacological analysis as well as recording of neuronal activity, the autonomic nervous system is subdivided into the sympathetic, parasympathetic and metasympathetic parts. The metasympathetic system includes the complex of microganglia situated in the walls of visceral organs and having marked motility activity (the heart, ureters, intestine, stomach). According to the localization, separate areas of this system can be defined by its site (for example, the cardiometasympathetic area in the heart). The metasympathetic system attains the features of true autonomy. Its cells have no direct connection with the high centers: this connection is established through mono- and polysynaptic sensory and afferent units of sympathetic and parasympathetic nature.


Source



Full article here

How is it different from sympathetic &amp; parasympathetic systems?

One major difference is in the location.

The Metasympathetic system 


  includes the complex of microganglia situated in the walls of visceral organs and having marked motility activity (the heart, ureters, intestine, stomach).


while Sympathetic system


  consists of cells with bodies in the lateral grey column from T1 to L2/3. These cell bodies are "GVE" (general visceral efferent) neurons and are the preganglionic neurons. There are several locations upon which preganglionic neurons can synapse for their postganglionic neurons:


and Parasympathetic system


  consists of cells with bodies in one of two locations: the brainstem (Cranial Nerves III, VII, IX, X) or the sacral spinal cord (S2, S3, S4). These are the preganglionic neurons, which synapse with postganglionic neurons in these locations:


For detailed location, see this image: