Because inside that buffered system, we have this acid, an equal concentration of its conjugate base. So if this is our acid, then the concentration of the conjugate base is exactly the same. And for this particular graph, the acid is once again acetic acid, and the conjugate base is the acetate ion. And so when we add the H plus ions into our mixture, some of those conjugate based molecules, the acetate ion reacts with those H plus ions forming acetic acid. And that decreases the amount of H plus ions. It consumes, some of those H plus ions so that a PH does not change as drastically as it does in this particular case.