Harry Potter and the Deathly Hallows follows the formula that Rowling uses for all of the HarryPotter books (and that many authors of book series use in order to maintain their "brand" identity) – all seven of the titles in the series are Harry Potter and the (Blah Blah Blah). Rowling's titles skillfully seize upon the central mystery of each of the novels, so that her readers, and often her characters as well, are kept guessing at their meaning. This is especially true from Book 5 (Harry Potter and the Order of the Phoenix) onward. In this case, the "Deathly Hallows" are the three mysterious objects at the heart of the story. Harry, Ron, and Hermione have the incredible task of figuring out what these are and how to use them in order to defeat Voldemort – which is to say, to save the world.
In a more poetic sense, we might also read more into the "Deathly Hallows" portion of the title; "hallows" are relics or holy objects, and "deathly" – well, y'all can figure out what that means. This title alerts us to the fact that there's perhaps more at stake here than the usual, run-of-the-mill magic objects; there's an air of sacredness and gravity here that suggests that this installment will really be a matter of life and death.
Rowling played around with two other possible titles as well: Harry Potter and the Elder Wand and Harry Potter and the Peverell Quest (source). Which of the three do you like best? (In our opinion, she could call it Harry Potter and the Crumple-Horned Snorkack for all we cared – we'd still be lined up at the bookstore at midnight!)