Buffers are compounds that resist changes in pH. Buffering capacity= Moles of acid or base added in buffers solution / change in solution's pH. The greater the buffering capacity, the greater the quantity of acid or base which must be incorporated with a material to alter the pH.
In general capacity is the highest for pH=pKa (regardless of whether you add acid or base).
the weak acid is acetic acid. the definition we are using is maximum amount of either strong acid or strong base that can be added before a significant change in the pH will occur