Jury nullification means "cancelling by jury".
It means that jurors – the people that sit in jury – can say that some law is against the Constitution or against justice. If that law is against these things, then jurors can free a man in trial (find him not guilty).
Jury nullification gives jury the power to "judge" the laws that Congress has written. In other words, the jurors are not only testing the man in trial, but also the law itself.