A "matrix" refers to the arrangement of symbols on the reels of a slot. The matrix determines how many symbols appear on any given reel and how they are arranged. Depending on the slot, the matrix can differ, but usually consists of three or more reels that are all equipped with a certain number of symbols. When you press on the spin button, the reels will spin showing randomly selected symbols. The matrix affects the odds of winning because it determines which combination(s) of symbols will count as a win. It is important to note that the matrix can differ from game to game and that a given game might have multiple matrices used for different bonus or free spins features.