During the course of the play, Hamlet grows and changes. Early in the play, he is filled with anguish about whether God is real (he questions whether death id oblivion or there is an afterlife). He also wonders whether the ghost is his father or an emissary from Satan, and whether or not he should take his own life. He feels the weight of the world resting on his shoulders, and he believes that all difficult decisions are up to him.

His whole perspective changes during his voyage to England. He believes that it is nothing short of providential that he was able to open and replace the letter calling for him to be killed the minute he set foot on English soil. He comes back to Denmark much more at peace because he is now convinced that God has his fate firmly in his hands. He can rest easy, because a good God is in control, as is illustrated by his allusion to the gospels, when he states that

There’s a special

providence in the fall of a sparrow.

The quote

If it be now,

’tis not to come. If it be not to come, it will be

now

suggests that Hamlet has put his fate completely in God's hand. He trusts God to have appointed the right time for his death. In the quote, Hamlet says to Horatio that if "it" [death] is to be now, all it means is that he won't die at a later date. If death is inevitable, it's inevitable, and so it doesn't matter whether he fights Laertes or not.

In a very convoluted way, Hamlet is expressing in these lines the conviction that destiny is inescapable. There's simply no point in defying it or trying to delay the inevitable. Horatio's been trying to talk his good friend Hamlet out of engaging in a duel with Laertes. He suggests that he should try to get out of it by pretending that he's ill.

But Hamlet's not about to listen to Horatio's advice. He believes that everything that happens in this world, whether it's the fall of a sparrow or a sword fight, is destined to happen. That being the case, it seems foolish to try and delay the inevitable; Hamlet must fight the duel, even if it should result in his death.

If death is going to happen now, then that means it can't be delayed; it won't happen in future. Instead of worrying about when death will happen, it's far better to be prepared for it when it comes, and Hamlet decides to embrace this philosophy rather than worrying about the imminence of his own death. What Hamlet is doing here is focusing on those things we can control instead of those things that we can't.

Hamlet is telling Horatio that it is not his time to die.

Hamlet has been through a lot by this point.  His father died, and then appeared to him as a ghost and demanded that he get revenge on his uncle, Claudius.  Hamlet angered his uncle by concocting the scheme with the play and got himself sent away, where he was supposed to be killed, but he escaped and turned the tables on his would be assassins. 

Now, he is supposed to fight a duel with Laertes, whose father he killed (one could argue he is responsible for his sister Ophelia’s death too).  Horatio tries to talk him out of it, telling him to say he’s sick, but Hamlet explains that what will happen will happen.

We defy augury. There’s a special

providence in the fall of a

sparrow. If it be now,

’tis not to

come. If it be not to come, it

will be now. (Act 5, Scene 2)

Hamlet is basically saying that he is not listening to any fears that he might die (that’s the reference to augury, which basically means prediction), and he is going to the fight.  If he is going to die now, he is ready to die now.  He makes an allusion to the “special providence in the fall of a sparrow” as if he expects divine intervention.  However, he goes on to say that “the readiness is all” meaning that he is ready to fight, and he will fight.

Hamlet does say “if it be not now,/yet it will come.”  He knows that he might die, and he is ready for that too.  He is confident, but he is a man on a mission.  He believes that he must do this, because he has made an oath to his father (actually his father’s ghost) and to himself, to see this through.

Although Hamlet says he defies the predictions, he really is about to die.  His uncle has betrayed him with this duel.  He cannot but see this coming.  When he is talking to Horatio here, he is not afraid to die.  He has already confronted his demons when he asked himself what happens when "we have shuffled off this mortal coil" (Act 3, Scene 1).  Hamlet is prepared to die, but he will take Claudius with him.