This is a common epilepsy warning shown before the start of some TV shows, movies, videogames etc. where the content may pose a health risk to certain viewers i.e. those who suffer from epilepsy. 'Flashing' means a bright light which changes from 'off' to 'on' rapidly, like the lights sometimes seen in a disco.

Flashing images quickly appear and disappear numerous times. Just like the flash of multiple cameras. They warn you about flashing images because it can induce seizures in people with epilepsy.

Well said, Chris. For individuals susceptible to seizures, flashing images can increase their vulnerability to them.

It most likely means images that quickly appear and disappear.