It's one of the terms used to describe the eyes-closed visuals one can get on a psychedelic trip.

It's a bit of a misnomer in this case as light helps induce it but the effect is similar.

It sounds like the state I've been able to get myself into a few times when I've been super tired, and then put some headphones on, close my eyes and focus on a central light in the dark. All the while listening to edm similar to Stutter house and other binaural beats. And that's it.

The first time I did this unexpectedly was a trip. I can't do this at will, and ignored it for a long time. I've recently started trying to see if I can control into it. I come out of this in an alert and super-relaxed state when I can get into that state.

edit: Forgot to mention that those "phosphene visuals" are exactly how I know I'm in this state.