In this last opus J.K. Rowling brilliantly puts an end to the Harry Potter story which had started 7 books ago.
This last episode tells how Harry and his fellows try to complete the mission that Dumbledore has left to them. This misson consists in the destruction of the seven Horcruxes in which Voldemort has sealed his soul. But in order to achieve this they have to be very careful because Voldemort has taken power of the Ministry of Magic and eagerly wants Harry Potter to be caught to fulfill his revenge. This duel between good and evil will only stop at the very end (which I obviously won't give away).
The title refers to an additional quest for Harry who has to choose between personal comfort and moral duty to the community. The deathly hallows are three gifts of Death that give their possessor enough power to beat Death herself! These three objects are the invisibility cloak, the elf wand and the resurrection ring.
To those who think Harry Potter is just another tale for children about wizards and witches I would like to say: read it and you will see how many parallels there are with our 'normal' world and how clever the comparisons are. I will quote some examples, knowing that many people are like St Thomas: Voldemort compared to Hitler in the quest for "pure-blood", again Voldemort, putting his soul into material things like a fervent capitalist, the quest for the elf wand which is a parallel to the cold war during which both sides struggled for the best weaponry, and so on.
This multi-interpretationality and intertextuality is, I think, what makes the whole Harry Potter sequel reading worth, it makes its richness and interest and the narration, improving identification and keeping up suspense till the last word, raises this work to the top of nowadays literature.