Tombstones are to remember the tribesmen who have died. Once a tribesmen has died they will be replaced with a tombstone.
If you choose "No" to keep the tombstone, you can then click and drag it to a new location.
If you click "Yes", the tombstone will sink into oblivion. There is no way to get it back.
If your whole tribe travels to another island, the tombstones will stay on the previous island.