What is a buffer solution?

A buffer solution resists changes in pH. This is possible because a buffer solution is a solution that is made up of a weak acid and its conjugate base. It can also be made up of a weak base and its conjugate acid. When you add a little bit of acid or base, the conjugate acid base pair react with it and therefore the pH is barely affected. There is also the Henderson-Hasselbach equation:

pH = pKa + log ([conjugate base]/[weak acid])

A good buffer is one where the pH is ±1 of the pKa.