A neutralization reaction is the process between an acid and a base (there are a number of different ways to define acids and bases). An acid is a compound, which dissolves in water by releasing H+ ions, and a base is a compound, which dissolves in water by releasing OH- ions (by Arrhenius' definition, the simplest one). In this case, the neutralization reaction is the process between HCl (hydrochloric acid) - an acid, and KOH (potassium hydroxide) - a base.