According to the arrhenius theory a substance that yields hydrogen ions as the only ion in an aqueous solution is

According to Arrhenius theory of acid and bases, an acid is a proton donor, while base donates hydroxyl ions. Further, acid and base reacts to form salt and water.
Consider following example.
HCl   +     H2O        →      Cl-         +      H3O+
here, HCl has donated a proton, hence it is an acid.
Now consider following reactionHCl     +      NaOH           ↔     NaCl      +        H2O
in this reaction, HCl donates proton, hence it is acid. On other hand, NaOH donates hydroxyl ion (i.e. OH-), hence it is a base.