When Claudius and Laertes conspire to kill Hamlet, they plan three separate methods of attack in order to ensure his death.
The first part of the plan, which Claudius suggests, is to goad Hamlet into a fencing match in which Laertes may kill him. This, Claudius says, will be just revenge for Hamlet's murder of Laertes's father, Polonius, which was also carried out with a sword. Fencing matches of this sort were not meant to be dangerous, and both parties would normally be dueling with dulled blades to prevent injury. However, Laertes plans to secretly use a sharpened sword so that he may wound Hamlet.
Laertes devises the second part of the plan, proposing that he will anoint the end of his sword with a strong poison. Thus, Laertes doesn't need to win the duel to kill Hamlet—he simply needs to strike him once. As he says,
I'll touch my point
With this contagion, that, if I gall him slightly,
It may be death.
To "gall" Hamlet is to make contact with him. So, if Laertes even scratches Hamlet with his sword, he will still be killed by the poison.
The third part of the plan is a contingency thought up by Claudius. He wants to make sure that this plan is entirely failsafe and not leave their success entirely up to Laertes's skill with his sword. He tells Laertes,
When in your motion you are hot and dry—
As make your bouts more violent to that end—
And that he calls for drink, I'll have prepared him
A chalice for the nonce, whereon but sipping,
If he by chance escape your venom'd stuck,
Our purpose may hold there.
He intends to poison the chalice that Hamlet would drink from when he becomes thirsty during the duel. Thus, even if Hamlet proves such a skilled swordsman that Laertes is unable to touch him with his poisoned weapon, Hamlet will still be poisoned when he pauses to drink from his cup (something he will undoubtably do during an exhausting duel).
In summary, provided Claudius and Laertes can successfully goad Hamlet into a duel, they have set up three possibilities for his death: he may be killed by a fatal blow from Laertes's sword, a scratch from the poisoned sword tip, or a drink from the poisoned chalice. However, in placing so many hidden weapons and threats around the room, Claudius and Laertes also allow for the possibility of far more deaths than Hamlet's—including their own.