Davy Jones' Locker, also referred to as the Land of the Dead, was a dimension to which souls claimed by the sea, or devoured by the Kraken, were sent. Though it was sometimes used as a term to refer to the bottom of the ocean, Davy Jones' Locker was a barren wasteland. People imprisoned there are technically not dead, but they cannot die. This is why Tia Dalma couldn't bring Jack Sparrow back to life–like she did with Hector Barbossa–as he was trapped, not dead.