What is meant by: Who would there fardels bear, to grunt and sweat under a weary life, but that the dread of something after death.

This quote from Hamlet occurs in Act
3, scene 1, and is part of the most famous soliloquy ever written:  "To be, or not to
be."  In this soliloquy, Hamlet is questioning why we continue to live when dying would
put an end to our misery.  Your particular quote expresses this sentiment quite well. 
In the quote, "fardels" is another word for burdens.  Hamlet is asking why anyone would
bear the burdens of a long and weary life full of suffering and toil.  He continues to
answer his own question:  we do not commit suicide because we are afraid of the
afterlife, the unknown, the

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undiscover'd country from whose
bourn

No taveller returns . .
.

And makes us rather bear those ills we
have

Than fly to others that we know not
of?

Throughout the soliloquy
Hamlet describes living as something to bear.  Hhe calls long life a "calamity" while
death is referred to much more positively as a "quietus" or a "consummation devoutly to
be wished."  The choice of "fardels" with its negative sounds supports the idea that
life is torturous and painful and poses a sharp contrast to the words he chooses to
describe death.  This diction lends credence to his argument that we bear the burdens of
life because we don't know what will happen after death.