He talks about them throughout his Robots and Empire series, maybe in Foundation as well. I wish he’d have given more detail about them, but that’s typical of Asimov, he was always introducing stuff and you’re supposed to imagine how it works.
It seems like the term is literal, and they are basically mashups, where you watch it like a movie but it has subtitles and maybe a voiceover. I’m speculating, though.
Maybe this might give you some ideas, at least. This one seems to have the best detail, and I pasted some others below: https://books.google.com/books?id=7pdVUgBL6Z4C&printsec=frontcover&dq=asimov+naked+sun&hl=en&sa=X&ved=0ahUKEwi20bui5KrKAhXHOCYKHZ5kD7IQ6AEIHTAA#v=snippet&q=book-films&f=false