Paper/rock/scissors is a game of chance between two people. On a signal you each make one of 3 shapes with your hand – rock (a closed fist), paper ( a flat hand) or scissors (a fist with two fingers making a V). The outcomes are that you make the same shape or a different shape. If the shapes are different then paper covers rock (paper wins), rock crushes scissors (rock wins) and scissors cut paper (scissors wins).
This game began in China and Japan and is a simple way to select who goes first, for example, in another game. It is like flipping a coin, throwing a die or drawing a straw from a pile. It is a selection method.
There is even an official website for discussing the rules of this game.