Answer is: the HCO3⁻ to act as a base and remove excess H⁺ by the formation of H₂CO₃.
Dissociation of carbonic acid: H₂CO₃(aq) ⇄ HCO₃⁻(aq) + H⁺(aq).
Adding acid: HCO₃⁻(aq) + H⁺(aq ⇄ H₂CO₃(aq).
A buffer can be defined as a substance that prevents the pH of a solution from changing by either releasing or absorbing H⁺ in a solution.
Buffer is a solution that can resist pH change upon the addition of an acidic or basic components and it is able to neutralize small amounts of added acid or base, pH of the solution is relatively stable.