This is a condition seen with persons who have Multiple Sclerosis and Optic Neuritis.
Normally,when your doctor shines a light into your eye, then the pupil contracts. Also, because of afferent nerve connections, shining a light on RIGHT will make the LEFT constrict and vice-versa.
With a Marcus Gunn pupil, the light does not constrict the pupil on either side.
For example, if the RIGHT eye has a Marcus Gunn pupil, then light shined on it will neither constrict the RIGHT nor the LEFT. Instead, the affected eye dilates to light.