The Carnelian minefield was a location populated by mines.
Sometime before stardate 41148, colonists sent a distress call which was received by two Federation starships. One of the ships was commanded by Captain Jean-Luc Picard, and the other ship was the assignment for Natasha Yar. During this rescue mission, Picard witnessed Yar risk her life to rescue a wounded colonist from the minefield. Picard was so impressed with Yar's bravery that he made a request for her to be assigned to his next command, the USS Enterprise-D. The captain of Yar's ship owed Picard a favor, so the request was granted.
A few years later, when responding to a charge of cowardice leveled against her sister by Ishara Yar, Picard recounted what happened at this minefield to prove that Natasha was not a coward. (TNG: "Legacy")