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.