For those who still remember movies in theaters — you might be familiar with the green screen before previews that reads “This preview has been approved for all audiences”.
Red band trailers instead have a red screen that displays a message:
“This preview has been approved for RESTRICTED audiences”.
Like an R rated movie this content is intended for a more mature audience and generally contains more graphic violence, language, sex, drugs, etc.
I don’t know that I’ve ever seen one in a theater. But they’re definitely popular and fun on the internet.