So at times zero before our reaction even occurred. We don't have any of our products yet the products haven't formed, so our Y is zero. Now, our X, the concentration of X of our reactants is at its maximum because none of this guy has converted to Y yet. However, as the reaction begins to proceed, the concentration of X begins to decrease, while the concentration of Y begins to increase because some of this X is converting or becoming Y. Eventually a point is reached in which the concentration of the reactants of X and the concentration of the products Y does not change. And this is known as chemical equilibrium.