Vertebral subluxation is a chiropractic term.  It literally means less than a dislocation.
The term describes a vertebra that is out of position or not moving properly. The human body perceives the subluxation and creates muscle spasm around the joint that is out of position.  The subluxation also affects the spinal nerve on the side of the vertebra.  The nerve signal is reduced by the subluxation.  The reduced signal results in reduced function in the cells and tissue. Therefore vertebral subluxation results in muscle spasm and reduced innervation.  This affects and reduces the ability to the body to heal and regulate.