The human body perceives its natural location in a space through 2 independent but interconnected organs:
Vestibular system - we can simplify it for a sensory system providing information about motion, position, and spatial orientation of the head. The detection is based on a liquid behaviour in an inner ear.
Visual perception - we can simply express it as a signal from eyes, from which the brain derives information about the motion, position and spatial orientation of the head.
At the moment when the perception coming from these 2 systems is different, the body does not know exactly what is happening and how to react - there is a so-called motion sickness. In virtual reality, it is very close to experience this state - typically the user watches a dynamical image through the VR headset (visual perception ✓) while his body is at a rest (vestibular system X).