Cysts in the back of the neck are typical “ganglion” or “synovial” cysts (the same thing). These are generated by degenerative facets, the joints in the back of the spine. If the cyst is not compressing the spinal cord or roots, it can be safely left alone and watched. However, this cyst must be causing you some symptoms or you would not have an MRI to determine what your symptoms derive from.
Some cysts will enlarge over time and need to be watched. If there is compression of neurological structures, a posterior decompression is the most common surgical procedure performed (unless there is a vertebra that is slipped).