Acetic acid, HOAc, is in equilibrium with the ionized form.

Because it is an equilbrium, changes in the concentration
of any of the three species results in changes to the others.

Buffers work by having a high concentration of a weak acid
such as HOAc that can react with added base.  Buffers also
have a high concentration of the corresponding anion, OAc-,
that can react with added acid.

Acetic acid alone does not make a good buffer because the acetate
ion concentration is so low that even a small amount of strong acid
will increase the acidity.

Let's make a solution that has high concentrations of both acetic acid
and sodium acetate.