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).
Jurors are regular, everyday people – not like judges, who are placed by the government. Because of that, there is an idea saying that jury nullification protects the rights of every person in the country, from bad government laws.