Buffer consists of undissociated acid (ha) and the ion made by dissociating the acid (a-). How does this system buffer a solution against decreases in ph?]

In buffer solution there is an equilibrium between the acid HA and its conjugate base A⁻: HA (aq) ⇌ H⁺ (aq) + A⁻ (aq).

When acid (H⁺ ions) is added to the buffer solution, the equilibrium is shifted to the left, because conjugate base (A⁻) reacts with hydrogen cations from added acid, according to Le Chatelier's principle: H⁺ (aq) + A⁻ (aq) ⇄ HA (aq). So, the conjugate base (A⁻) consumes some hydrogen cations and pH is not decreasing (less H⁺ ions, higher pH of solution).

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