It's a device that sends a rapid burst of air mixed with water/mouthwash into the places that you normally floss. It's quick, easy and very very effective.

I think security were unhappy as they considered it a mini air compressor. I have no idea if that's what it does, but I wasn't going to give it up without a protest. They relented in the end, but since then I've often reverted to interdental brushes on my short trips just in case.