A buffer solution resists (or buffers) a change in its pH. That is, we can add a small amount of an acid or base to a buffer solution and the pH will change very little.

Sodium acetate (CH3COONa) dissociates into ( Na⁺) and acetate ion (CH3COO^-) in water

CH3COONa <> Na⁺   + CH3COO^-

Formic acid (HCOOH) dissociates into H⁺ and COO^-

HCOOH <> H⁺ + COO^-

since the salt is completely ionised, it provides the common ions CH3COO– in excess. The common ion effect suppresses the ionization of formic acid. This reduces the concentration of H+ ions which means that pH of the solution is raised.

Yes,  solution of HCOOH CH3COONa s a buffer.