Photophobia is fear of or aversion to light. Because squinting and screwing up our eyes in strong light, especially sunlight, is normal in humans, many people assume that this is also normal in dogs. However, this response to conditions such as sunlight, fluorescent light or white cloud cover is not normal in dogs. It is almost always a sign of pain (as of course it can be in humans). This underlying pain can be caused by a number of conditions including cataract, glaucoma, infections, meningitis, conjunctivitis, dry eye or neurological conditions, including, significantly for Cavalier owners, dilated ventricles caused by Chiari Malformation. Many of these conditions are hereditary in Cavaliers, so any sign of pain in the eyes should be taken seriously.
If your Cavalier reacts to strong light in this way, you should discuss possible causes with your vet and perhaps get a referral to an ophthalmologist for a more in-depth examination. A video of your dog reacting badly to light can be very helpful for a diagnosis.