It's energy in the Y axis versus time or progress or reaction on the x axis. Now, this black curve is the curve that represents before additional catalysts. Notice activation energy goes all the way up to this blue level. Now, when you add that catalyst, what happens is that activation energy is lowered by this much to this red level. And that means more molecules, on average will have enough kinetic energy to climb this new activation barrier and form the product. And that's exactly what happens when you add a catalyst.