It’s a treatment in the spa villas, I believe only on the Magic and Wonder. For some reason the description is no longer on the website, but if it’s available for your cruise it will show up under spa activities when your booking window opens.
I think you have to do a couple’s treatment in the villas on Fantasy and Dream (which seems dumb to me, but since the couple’s experience is a lot more $, they probably want to get as much profit as possible from the villas).