Answer:
In short it means that the case was dismissed inconclusively. There may have been missing evidence or procedural requirements not met, and therefore the case could potentially be brought before the court again by the parties involved.
It is an order issued by a judge actually dismissing the charges that were brought in the case - usually for some legal insufficiency, or lack of evidence, in the case itself. Dismissal WITHOUT Prejudice means that the charges MAY be re-instituted and brought before court again AFTER the legal insufficiency is cured.
It is usually a term used in the context of case being dismissed. The significance of it is that the Plaintiff/Prosecution retains the right to re-file the lawsuit a second time.
A case can be dismissed without prejudice by the judge (such as, if he or she grants a motion to dismiss filed by the defendant--but in such a case, the plaintiff/prosecution usually will be given the right amend the complaint (restate the allegations of the lawsuit), or it can be voluntarily dismissed altogether by the plaintiff/prosecutor. In many jurisdictions, a second voluntary dismissal by a plaintiff will preclude any subsequent refiling of the cause of action, but that is governed by local law.