ANSWER
It is an acid because it increases the concentration of hydronium ions.
HCl is an acid. When HCl is added into water, it undergoes the following dissociation:
HCl + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq).
The Arrhenius definition defines acids as subtances that release H⁺(aq) when put in water. This H⁺(aq) ions that HCl releases reacts with water to form H₃O (hydronium), leading to an increase in concentration of H₃O.