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To determine whether or not an accused individual is guilty there are two primary options in the USA. A trial before a judge who makes the decision, or a trial by a "jury of your peers" where the whole jury must agree that the individual is guilty. A jury of one's peers means that the people selected to hear the case are selected from the general populace and have no substantial connection to the accused. For example, you wouldn't put the person's mother on the jury. The jurors are not required to be lawyers or experts in any field. Just average people.

If you just wanted some people to take the facts of the case and the facts of the law and determine whether or not the accused was guilty, then you would want experts and lawyers on the jury. That's how trials used to be hundreds of years ago. A judge, often appointed by a king, would pass sentence over the peons brought before him. Since our legal system has average everyday people as jurors, clearly they are supposed to do more than that.

This is where jury nullification comes in. The jurors not only judge based on the facts of the case, but also on whether or not the law in question is just. If an individual is accused of a crime, and is clearly in violation of the law, the juror can still find them not guilty if the law in question is unjust. In essence, the jurors nullify the law by refusing to convict. For example, during the prohibition era, it was not unheard of for juries to return not guilty verdicts for people accused of selling or transporting alcohol. The jurors thought the laws ~~was~~ were wrong so they refused to convict. A much more tragic example was in the deep south where jurors would sometime refuse to convict people of lynching black people.