Not being sure of what the term meant, I looked it up:
the acquitting of a defendant by a jury in disregard of the judge's instructions and contrary to the jury's findings of fact
NOTE: Jury nullification is most likely to occur when a jury is sympathetic toward a defendant or regards the law under which the defendant is charged with disfavor. Except for a statutory requirement to the contrary, a jury does not have to be instructed on the possibility of jury nullification.
However, what surprised me is the fact that someone can be arrested for handing out literature about jury nullification. Turns out they chose to charge Julian Heicklen with jury tampering (though anyone with a brain can see he wasn't doing that). You see, juries are part of the court process as part of checks and balances. The reason we have juries is it was figured that a group of individuals who were chosen at random, and for such limited amount of time that it would be difficult to corrupt them (I say difficult because it has happened), would be better able to render a fair verdict.
I remember growing up hearing a lot about the prohibition era. However, what I was never taught during those classes is that much of the time, juries refused to convict not because they had reasonable doubt, but because they felt the law itself was unjust. This practice was largely responsible for the prohibition era coming to an end.
You see, if enough juries aren't willing to convict because they feel a law is unjust, the government will (hopefully) figure out that arresting people for such a crime is a waste of resources. What I refer to is things like drugs, gambling and prostitution. Now, while I have my own belief about these things, I have a stronger belief that the it is not the government's place to prohibit people from engaging in activities which don't harm other people.
I have begun reading the Citizen's Rule Book by Charles R. Olsen, a World War II Marine veteran. There is a pdf version you can read here