The Dancers are pillars of meteorite, magnetic iron, standing in a stone circle in the kingdom of Lancre. Periodically (about once every fifty years), the barrier between dimensions weakens, and the inside of this stone circle can lead to the wintry part of Fairyland, world of the Elves. They are roughly man-height, as wide as a fat man and look rather dull and unimpressive. In Fairyland however, the Dancers look a lot bigger. Though there are 7a stones, only three of them are named (the Piper. the Drummer, and the Leaper).
The Dancers were meteorites that crashed down long ago and were found and arranged around a weak point between dimensions by dwarfs, to keep the elves out. Since elves get ill in the presence of iron, the Dancers prevent elves from coming through to the Discworld.
Major role in Lords and Ladies.
Michael Moorcock uses a similar conceit in his novels: he calls it The conjunction of the Million Spheres, and uses it to explain why the walls of reality periodically overlap at sensitive places to form doorways in time and space, where things and people can leak from one or more plane of the Multiverse to another. This explains why the Gods of Law and Chaos can move between planes, and how several incarnations of the Eternal Champion can periodically meet up, fight a Quest together, and sell books for the Eternal Author.