A buffer is a solution that resists changes in pH when small amounts of acid or base are added. It must contain a weak acid and its conjugate base (or a weak base and its conjugate acid). When acid (H+) is added, the conjugate base absorbs it; when base (OH-) is added, the weak acid neutralizes it. The pH of a buffer is calculated using the Henderson–Hasselbalch equation. Buffers, like the carbonic acid system in blood, are essential for maintaining stable pH in biological and chemical systems.