For strong acids such as H2SO4 (Sulphuric acid) and HCL (Hydrochloric acid), H+ ions dissociate completely. For weak acids like CH3COOH (Vinegar, or otherwise known as ethanoic acid), the dissociation of the H+ ions is partial. (There is a dynamic equilibrium, H+ ions dissociates and form back into the reactant)
Since the concentration of H+ ion determines the acidity, it could be explained as such.
