It is like a little mini wooden chair, that sits on the normal adult chair and hooks over the back. Then has straps to hold baby in place.
I have used them several times in a couple of different cafes for DS, and he really sits well in them. I asked the waitress today. She did not know where they got them from and the owner was not around to ask.
I can't find it on google but am still looking (having a brain fart wrt what to search for!)