A Tombstone is a special interactive item created by the game when the player dies, at his/her last location.
After the player has respawned, they can return to the Tombstone and click it to recover a certain amount of the lost experience, inversely proportional to the player's level. The Tombstone then simply disappears.
It is not possible to have more than one Tombstone left by the same player: if the player dies with a Tombstone present without activating it, the old Tombstone will be replaced by a new one. The second tombstone will not have the experience from the first one; if the first one is lost, the experience goes with it.
BUG: If you click quickly, two times on your tombstone, you gain double the experience you have lost. (Maybe, the developers directly made it for the game, who knows...a minigame with bonus :))