Forum non conveniens is Latin for "inconvenient forum."
Although there are rules which govern where a lawsuit must be filed, sometimes the location is inconvenient for the witnesses or parties. If a party makes an adequate showing of inconvenience, the principle of forum non conveniens allows a judge to decline to hear, or to tranfer, a case even though the court is an appropriate court for the case.
E.g., Vince and Claire's divorce case was decided in Miami, Florida, but both have since moved to Orlando. Any request for modification must first be filed in Miami, but either party could request that the court decline to hear the case, and instead, transfer it to Orlando for the hearing.