The short description is it's a hallucination, the long description is that it's not a hallucination, but seeing more of what's there, all the time. Like seeing the white blood cells in capillaries on the surface of your eye lens, the wrinkles in the gel on the back of your eye, and triple imaging between both feeds from both eyes overlaying a consolidated stereo view. IME explaining visual snow syndrome (VSS) to a layman is a lot like trying to explain color to a blind person, so I keep it short.

There isn't much known about the mechanism that causes VSS/HPPD, but the current line in the sand that they draw for performing VSS studies is that HPPD is temporary (<6 months) and VSS is permanent. HPPD is long known to be caused by psychedelics, but the permanent HPPD is now being re-diagnosed as VSS since they're the same symptoms and activity in PET scans.

Unfortunately, there are not a lot of studies I can link to because they've only just started researching it these past couple years, but the neuro opthamologists and psychiatrist I work with have VSS patients who contracted the syndrome after using LSD or acid, with one as far back as the 70's.