In Act 3, Scene 2, Hamlet uses the players to perform The Murder of Gonzago (Hamlet refers to it as The Mousetrap because he views the play as a trap where he will catch "the conscience of the King.")
After watching the play, Rosencrantz and Guildenstern report to Hamlet that Claudius is furious. Guildenstern says Claudius is "with choler" which means "angry" and suffering indigestion: mentally and physically affected. Hamlet replies:
Your wisdom should show itself more richer to signify this to his doctor, for for me to put him to his purgation would perhaps plunge him into far more choler. (III.ii.279-81)
Hamlet uses the word "purgation" which holds two meanings: to physically and spiritually purge. Hamlet says that they should tell Claudius' doctor about his "choler" (mental and physical ailment) because if Hamlet were to truly purge Claudius of his crime (to expose his crime even more blatantly than with The Mousetrap), Claudius would be even more "with choler": more physically and mentally upset.
These notions of Claudius being mentally upset and/or physically ill foreshadow the moment in Act 3, Scene 3 when Hamlet attempts to kill Claudius. Since Claudius is praying, Hamlet postpones killing him. Hamlet says that "This physic but prolongs thy sickly days." (III.iii.96) The "physic" refers to a medicine and that medicine is Claudius' prayers. Claudius, by praying, extends his life. Since he still intends to kill Claudius, Hamlet calls the remainder of his life "sickly days" which suggests that Claudius' death is the result of of his sick mind: his criminal act of killing Hamlet's father.
Throughout Hamlet's plan for revenge, he wants to expose Claudius' guilt as dramatically as possible. Therefore, he wants Claudius to suffer mentally before he "physically" kills him. So, when Hamlet exposes Claudius' crime in this dramatic way, he will, in a sense, be purging it out of him (so that the public can see his guilt). Since this is when Hamlet intends to kill Claudius, it is at this moment when Claudius' "choler" (mental and then physical ailments: shame and death) will be the most severe. So, this line in Act 3, Scene 2 also foreshadows Claudius' death.