A "Petra" is a rock that can be found by using the Quarry (/quarry) command during a Ballista. Once a Petra is obtained, you can then toss it inside of a Rook to score points for your team. You are able to carry an infinite number of Petras; however, you can only score 5 Petras at a single time.
Please refer to the rules of Ballista for further information.