In these verses, Hamlet is using a Homeric simile (an extended or detailed comparison of several or more lines) to make his point that a single negative quality of a country or a person can cancel out, in the minds of others, their positive attributes. He says the following about the boisterous behavior at the Danish court:

This heavy-headed revel east and west

Makes us traduced and taxed of other nations.

They clepe us drunkards and with swinish praise

Soil our addition. And indeed it takes

From our achievements. . . .

So oft it chances in particular men

That for some vicious mole of nature in them. . . .

Carrying, I say, the stamp of one defect . . .

Their virtues else. . . .

Shall in the general censure take corruption

From that particular fault. The dram of evil

Doth all the noble substance of a doubt

To his own scandal.

In other words, a single fault or defect will spoil for others a multitude of virtues that person might possess, just as the Danes are apparently censured by other countries for a single traditional activity they carry out at court. The wider significance of this to the play can be seen in several ways, but the most striking, in my view, is that it is Hamlet's own way of looking at others that he is describing. In general, Hamlet views people and the outside world negatively because of just a single or incidental fault. His judgment of Claudius is justified, because Claudius is in fact a murderer. But Hamlet also has conflicts with almost all the other major characters except Horatio. He judges and condemns Polonius, Ophelia, his own mother, Gertrude, Laertes, and Rosencrantz and Guildenstern. Additionally, his negativity is extended to himself and to his own life. In the "to be or not to be" soliloquy he basically makes the judgment that there is so much evil in the world that it is logical to ask why a person would want to stay in this life—lacking the fear of death—when "he could his quietus make with a bare bodkin. / Thus conscience does make cowards of us all." His final point here is that all people are essentially cowards because they wish to stay in this life solely because the next world could bring something worse. These judgments made by Hamlet, as stated, are a kind of extension of the complex simile in act 1.

Hamlet here, as he does so often in this play, is expounding on the nature and behavior of man.  He says here that so often, men are born with a particular fault.  He admits that the fault is not to be blamed on the man himself, for no one can pre-choose the characteristics that make up his person.  However, this fault is so destructive that, no matter how good the man is otherwise, it will always taint the behavior and bring down the good name of that man.  Hamlet, here, is referring to Claudius, Gertrude, and himself, as well as foreshadowing the destruction of all three that the play will bring about.

Besides foreshadowing, Shakespeare uses many similes and metaphors:  Calling the fault a "vicious mole" of nature and a "dram of eale"; saying that even if a man be "as pure as grace"; referring to a man's intellect as a "fort" of reason. 

Shakespeare's complex studies of human nature, as especially portrayed in such plays as Macbeth and Hamlet, were forebearers of character-centered novels and plays.  Shakespeare was the ground-breaking writer that abadoned the larger than life epic stories for more realistic and multifaceted tales of the human condition, opening the door for such writers as Charles Dickens, Jane Austen, and the majority of 20th century authors.