According to Bronsted-Lowey concept of Acid and Base, an Acid is any specie which donates proton (H+) while, Base is a substance which accepts proton from acid. In the reaction above, Ammonium ion is acting as Acid and Hydroxyl ion as Base because Ammonium ion donates proton and converts into ammonia a conjugate base, while, hydroxyl ion accepts proton converting into water (a conjugate acid in this case).
