A buffer solution is a special type of solution that resists changes in pH when small amounts of acid or base are added.
It usually contains:
- A weak acid and its conjugate base, or
- A weak base and its conjugate acid.
Because both parts are present, the weak acid can neutralize added bases, and the conjugate base can neutralize added acids.
In short: A buffer solution keeps the pH nearly constant, even if you add a little acid or base.