This is one of my favorite poems by John Donne. It is meant to comfort those facing death. In the poem, Donne makes it clear that death has no real lasting power. It is but a temporary pause between this life and eternal life.

The lines you asked about specifically could be summed up like this:

Thou art slave to Fate, Chance, kings, and desperate men,
And dost with poyson, warre, and sicknesse dwell,

(You, death, are only a slave. Fate, Luck, kings and even desperate men control you and tell you when to appear. You are so low that you live with poison, war and sickness--not a very good place to dwell.)

And poppie, or charmes can make us sleepe as well,
And better then thy stroake; why swell'st thou then;

(Poppies are known for their sleep inducing qualities (morphine and other opiates are made from them, and many people have compared death to sleep) so here Donne is saying--Drugs and charms (maybe herbal) make people sleep as well as you can--and even better, so why do you, death, brag or feel pride in your ability? He goes on in the final line to say that death will die when people rise in eternal life.)

For the full text, see the link below.

You can also find information about Donne at the second link.