In a court of civil law, the phrase "dismissal without prejudice" means that the court has decided that it is unable to make a ruling based on the evidence. Dismissal without prejudice is not a final judgment, and the case might be retried in the future when more evidence is available. When a case is dismissed with prejudice, it implies that the court has decided that the plaintiff's case was unfounded, and the plaintiff is barred from further filing on the same issue.
A safety measure called the doctrine of res judicata has been put into place in the common law of most countries because of the possibility of harassment via the court system. This doctrine of res judicata, a Latin phrase meaning "a thing decided," is a prohibition on the court's trying the same case multiple times. Without this doctrine, an aggressive plaintiff could clog the court system dockets and harass defendants with multiple frivolous lawsuits on the same case. With the doctrine of res judicata, the court can decide whether a plaintiff's case is frivolous and then permit a retrial by filing a dismissal without prejudice or bar future proceedings in the matter by dismissing the case with prejudice.
In the majority of cases, when a case is dismissed because of a lack of evidence, the court will enter the filing as a dismissal without prejudice, thus giving the plaintiff another chance to gather evidence to prove the allegations. If a plaintiff has demonstrated overt aggression toward the defendant, such as committing perjury, providing misleading information or filing multiple unfounded and frivolous actions and lawsuits after a previous dismissal without prejudice, the court might decide that the plaintiff is attempting to harass the defendant through the legal system. When this happens, the court often decides to dismiss the case with prejudice, thus eliminating any chance for the plaintiff to have the case revisited in the future.
A dismissal with prejudice in a civil matter is essentially the same as an acquittal in a criminal court. It is the court's way of saying that there is not sufficient evidence to prove the guilt or wrongdoing of the defendant. It also is a final judgment based on the court believing beyond a reasonable doubt that the plaintiff is filing baseless lawsuits in order to harass the defendant. In civil cases where the court has entered a dismissal with prejudice against the plaintiff, it can be assumed — based on the merits of the evidence presented to the court — that the defendant is not guilty of the allegations that were brought against him or her.