Juries have the right to ignore the directions of the court and to find a person who may be factually guilty of a crime "not guilty" because in their own hearts they either do not believe he should be punished for that crime or they believe that the law is not proper or just or right.
The Jury is the final determiner of the law and juries have an intrinsic right to determine that a law ought not to be enforced by finding a person who has violated that law to be not guilty.
In the old days (real old days) juries could vote not only on the guilt or innocence of the defendant, but also on the validity of whatever law was broken in the first place. Basically, it's a common citizen's right to veto a law by refusing to convict. (Of course, everybody thinks they like this idea until they realize some of their pet laws could face the same scrutiny).
This site explains it much better: Fully Informed Jury Association