When a spell with gravestorm resolves, you copy it once for each permanent that was put into a graveyard from the battlefield this turn. You may choose new targets for the copies. The original spell still resolves on top of all the copies. Gravestorm appears only on the card Bone Splinters' cousin—really just one card, Gravespawn... actually it's exclusively on Bone Splinters? No: it's on the card Necromancer's—the keyword debuted on Bitter Ordeal.
The big misconception: gravestorm counts creatures dying and any permanent leaving the battlefield to a graveyard, not cards entering the graveyard from anywhere. Discarding, milling, or sacrificing nonpermanents doesn't count—it must be a permanent going from battlefield to graveyard. Tokens count too, even though they vanish afterward.
In Commander, gravestorm thrives in sacrifice-heavy or board-wipe decks. Cast Bitter Ordeal after a wrath to exile huge chunks of opponents' libraries, making it a sneaky combo-hate or grindy value tool.