In Shakespeare's Hamlet, what does Hamlet mean when he wonders "What is a man if his chief good and market of his time be but to sleep and feed?"

In Act IV, scene iv, of Shakespeare's
Hamlet, the Prince of Denmark's main concern is the purpose of man,
and how Hamlet fits into it.

Hamlet's
speech that begins with, "What is a man if his chief good and market of his time be but
to sleep and feed?" is speaking to the fact that if we are only on this earth to pass
time and eat, we are no better than the animals. To what end, then, is man put on this
earth? In it, Hamlet berates himself for not having done anything yet to avenge his
father's murder.

Hamlet then compares himself to Young
Fortinbras and his men. For a small piece of land, they are prepared to die. It is not
for the land itself, but for the honor Fortinbras perceives to have been lost when his
own father, Old Fortinbras, lost his land and his life battling with Old Hamlet.
Fortinbras is often considered a foil for Hamlet, who is someone with a great many
similarities: by comparison it would seem that Fortinbras is a more honorable man who
has acted upon his sense of duty and honor, while Hamlet vacillates about killing
Claudius while not doing so.

Hamlet, in this quote, is
wondering what separates man from the beast: what is it that makes life on earth
worthwhile? He, more than anyone, has more than enough reason to kill Claudius and he
does not. Yet, for a much smaller concern over honor, here Fortinbras and two thousand
men are ready to die for their honor. The land they fight for is not large enough to
bury them if they are killed, and yet for honor, they will die. And if
they are willing to do so for such a small prize—steeped in
honor—Hamlet feels honor-bound to finally avenge his father's death as a point of
his honor, so much more important than a tiny piece of
land.

...to
my shame I see the imminent death of twenty thousand men / That, for a fantasy and trick
of fame, / Go to their graves like beds, fight for a plot / Whereon the numbers cannot
try the cause, / Which is not tomb enough and continent / To hind the slain? O, from
this time forth / My thoughts be bloody or nothing worth.
(IV.iv.61-68)