Because if we begin to add H plus ions into our mixture because of some type of acid based reaction that takes place inside that solution, then the conjugate base that exists in the water can basically interact and consume some of those H plus ions, and that will decrease the amount of H plus ions in our mixture. And what that does is it creates a gradual change in PH instead of a drastic change in PH. And so if we examine the Titration curve for a buffering system, for example, if our acid is acetic acid and the conjugate base is acetate ion, we get the following curve as seen in this graph. So if we look at this box in region, instead of having a drop in the PH, we have a very, very gradual move. And so instead of adding a small amount to change our PH drastically, we have to add a lot of the acid to actually create the same drastic change in PH. So this relatively flat curve basically means so the flat curve means that our system is able to actually resist and prevent drastic changes in PH from actually taking place.