Solns of KNOWN pH that RESIST CHANGES in ph casued by addition of SMALL amounts of acid or alkali. I have stressed the important words to use....
Acidic buffers are made from a solution of a weak acid and its salt in the same solution. This mixture provides a large concentration of BOTH anion AND un-dissociated molecular acidic compound. The salt is therefore said to provide a reservoir of anions..
A weak acid on its own cannot act as a buffer becasue the standing concentration of anions is necessarily very small...BECAUSE the acid is "weak" and therefore ionised to only a very small extent.
Remember the expression that links Ka to the concentrations of [SALT], [ACID] and [H+] species....