This monument is the symbol of Iwagakure in a similar manner to the the Hokage Monument found in Konohagakure. It's comprised of an intricate pedestal with a simple, smooth stone on top of it. The pedestal itself is in a pool of water that is filled with similar stones.
Will of Stone
The "Will of Stone" (石の意志, Ishi no Ishi) is the Nindō that exists symbolically in the monument. Similar to Konoha's Will of Fire, the Iwa shinobi live their life by this great principle. The First Tsuchikage told a young Ōnoki that the stone holds no true significance to the monument, even going as far as to simply throwing it back into the pool with the other stones and replacing it with another random stone. He uses these stones to demonstrate how essential the strong will of the shinobi is to Iwagakure.