Dear student,
Acid-base buffers confer resistance to a change in the  pH of a solution when hydrogen ions (protons) or hydroxide ions are  added or removed. An acid-base buffer consists of a weak acid and its conjugate base (from a salt) or a weak base and its conjugate acid (from a salt). A buffer works because it contains a substantial amount  of a weak acid and a weak base (the conjugate acid-base pair) at  equilibrium with each other. When protons (from an external source) are  added to the buffer, some of the base component of the buffer will react  with the protons and turn into the conjugate acid (which is the  weak-acid component of the buffer) and thus neutralizing most of the  protons added. When hydroxide ions are added (or, equivalently, when  protons are removed from the buffer), some of the weak-acid component of  the buffer will dissociate and turn into the conjugate base (which is  the weak-base component of the buffer) thus replenishing most of the  protons removed.