The Chairs is set in the house of the Old Man and Old Woman. The crazy thing about this particular house is that it's surrounded by stagnant water as far as the eye can see. The fact that the water is stagnant could be seen as symbolic. The elderly couple's lives have in many ways become stagnant as well. The two don't really do much of anything. They just hang out in their house playing pretend games everyday; their lives have become an endless repetition of the same old thing. The water surrounding the house also serves to heighten the sense of the couple's isolation; they're totally cut off from whatever remains of the rest of the world.
Though the play never says so specifically, it's quite possible that it takes place at the end of the world. The couple says that the city of Paris, where they apparently used to live, no longer exists. Many Absurdist plays take place in this kind of seemingly post-apocalyptic universe. (Beckett's Endgame is another good example.) It could be that the Old Man and Woman are the last two people on earth, and that the entire world is now covered with stagnant water. These kind of apocalyptic landscapes were perhaps inspired by the devastation of World War II and the constant fear of nuclear annihilation that the conflict left in its wake.