Ham.
A man may fish with the worm that hath eat of a king, and eat of the fish that hath fed of that worm.
King
What dost thou mean by this?
Ham.
Nothing but to show you how a king may go a progress through the guts of a beggar.

What point does Hamlet seem to be making in this exchange?

You never know what you may be eating.
A beggar is as good of a fisherman as a king.
Death reduces everyone, even kings, to worm food.
A fish is not particular about the worms it eats.