Acid-base equilibrium refers to the balance between acids and bases in a solution, which affects the solution's pH level. Acids are substances that donate protons (H⁺ ions), while bases accept protons. The equilibrium is established when the rate of acid dissociation equals the rate of base association, leading to a stable pH. In biological systems, maintaining acid-base equilibrium is crucial for processes like cellular respiration and enzyme activity. The body uses buffers, such as bicarbonate, to resist changes in pH, ensuring that conditions remain optimal for various biochemical reactions.