"Photophobia" is a misleading term.
Despite the "-phobia" suffix, photophobia is not a "phobia" or morbid fear along the lines of claustrophobia or arachnophobia. Instead it is a medical condition in which one's eyes are simply more susceptible to light than the average person. Some photophobics prefer the term "photosensitive" to avoid stigmas associated with "-phobia" words, though "photosensitivity" actually means different things entirely.
There are lots of possible causes and manifestations of photophobia. In my case, the irises of my eyes simply tend not to close as much or as quickly as they should in response to bright conditions, thereby letting in more light than they're meant to. This can result in discomfort, and temporary vision problems.
For comparison, think of what it's like having a bright flashlight or spotlight shone directly into your eyes. Imagine mundane levels of ambient light having that same effect on you.
Thankfully my particular case is not severe or debilitating, it's just a pain sometimes. Under normal circumstances bright daylight is the most intolerable form of light for me, but the more cloud cover the better I manage. I also take quite a bit longer than most people to adjust to a suddenly brighter environment; for example going into a lit building at nighttime, or switching on the lights in a dark room. Most people will take a couple of seconds to blink and adjust; in my case it can be a matter of minutes, more with certain types of fluorescent lamps. I'm also more susceptible to glares at night from things like bright headlights and streetlamps.
I manage thanks to my collection of cool shades. This is why you might occasionally see me wearing sunglasses when others might not, such as indoors and/or at night.) As a side benefit, cool shades are cool. I like cool shades! Who doesn't like cool shades?