Light Sensitivity is a property possessed by some entities. An entity with Light Sensitivity treats lit illumination the same as shadowy illumination in combat scenarios, or as dark illumination in exploration scenarios.
Light Sensitivity is often combined with another visual property. An entity with both Light Sensitivity and Low-Light Vision simply swaps the rules for lit and shadowy sections in combat. An entity with both Light Sensitivity and either Darkvision or Sightless Vision treats lit sections as though they were shadowy and shadowy and dark sections as though they were lit; in exploration, this combination simply swaps the rules for lit and dark regions.
See also: Sight