Yes, that's a bubble axis that you can slot into the hotshoe of your camera to show everyone that you have problems leveling your horizon. I have a bubble axis on my Benro tripod and every once in awhile I look at it and think: what is that? No, I never use it. I don't understand how someone can't tell their horizon is not level from looking through their viewfinder. Okay, some cameras have extremely crappy viewfinders; but those are on point and shoots and really you are unlikely to be using that for your spot on landscape. To be fair, I have shot my share of landscapes where the horizon is off -- well, like the comment in the link says: I just crop it.

Thanks Jhayne for the link.