Original: In the chapter "King's Cross", Dumbledore's spirit tells Harry that when Voldemort failed to kill baby Harry and disembodied himself, Harry became an unintentional Horcrux; Harry could not kill Voldemort while the Dark Lord's soul shard was within Harry's body.
Simplified: In the chapter "King's Cross", Dumbledore's spirit tells Harry that when Voldemort failed to kill baby Harry and disembodied himself, Harry became an unintentional Horcrux; Voldemort could not kill Harry while the Dark Lord's soul shard was within Harry's body.