LOP stands for Lack of Prosecution. Generally Dismissal for lack of prosecution occurs when the court closes a case as nothing has been filed within a specified period of time. The court presumes that as there is no record of activity the party does not wish to pursue the case. The time of inactivity varies according to state laws. For example lack of prosecution can occur when no service of process has been made and certified to the court within the time specified or when no responsive pleadings has been filed and the plaintiff has not moved for entry of default within the time specified, after the time for filing responsive pleadings has expired.
Usually the case is dismissed without prejudice so that the party can re-file the case.
Example of a State Statute (Pennsylvania) on lack of prosecution
E.D. Pa. Handbook COURTROOM DEPUTY CLERKS says
***After a case is filed, the courtroom deputy clerk checks the docket for timely service of process and the filing of an answer. If service has not been made within 90 days, a letter will be sent by the courtroom deputy clerk asking that service be made by the 120th day. If service has been made but the complaint has not been answered, again a letter will be sent by the courtroom deputy requesting counsel to motion for judgment by default. Please do not ignore these notices. If you do, it could result in dismissal of the case for lack of prosecution.