It means NOT PROSECUTED.
Nolle Prosequi, a latin phrase meaning "unwilling to pursue", is the term used in many jurisdictions to describe the prosecutor's voluntary dismissal of criminal charges. Generally the request to "nol pros" a case is made after the filing of an indictment or information, when the prosecutor has determined that adjudication of the charges is not in the interest of the government.
In most circumstances the court's permission is required for the nolle prosequi to be valid.