A voluntary dismissal is the end of a court action due to a request being made by the plaintiff in the action. If the plaintiff takes the case and then ends it themselves, it would be considered voluntarily dismissed because the only person that dismissed it is the one that initiated it in the first place.
A voluntary dismissal can only be made prior to an answer being submitted by the defendant. Only at the outset of the case does the plaintiff have a right to call it off. This is subject to the Federal Rules of Civil Procedure, namely Rule 41 (a).