The title of Aldous Huxley's novel, Brave New World, refers to a poem within the novel itself. The poem, is speaking of beautiful things, when John first brings up the poem, he is speaking about Lenina. John and Lenina just met, and he is mesmerized. He thinks she is as beautiful as anything he has ever seen, which is why he quotes the poem.
John, "The Savage" says, "O brave new world!" multiple times throughout the novel. The first time John says. "O brave new world!" is on page 139, when John is quoting, Miranda, The Tempest, From Shakespeare. The last time John says, "O brave new world!" is on page 210, when the soma rations were being given out, at the end of the book. The first time was about Lenina being so incredibly beautiful to him, the last time, was when John, is saying that the beautiful world had been ruined, and that maybe by quoting the poem, he might be able to change the world.
The title, Brave New World, is referring to a wonderful new world, that is perfect, and undisturbed. However as the novel progresses, it is apparent, that the world is not so perfect after all, so the meaning changes into, a world that will hopefully change into a wonderful one.