The reason for this is that aqueous KOH being alkaline in
nature gives OH− ions. These OH− ions then act as strong
nucleophile and attack on electrophilic carbon of alkyl halide
molecule. Thus the leaving group leaves and OH− substitute the halogen atom of alkyl halide molecule, forming alcohol as a product. Consequently, aqueous KOH gives substitution reactions.
On the other hand, alcoholic KOH(mostly having ethanol) produces C2H5O− ions which are stronger base than OH− ions. So these abstract the β- hydrogen of alkyl halide molecule, forming an alkene. That's the reason behind the statement; Alcoholic KOH give elimination reactions.
Hope it helps...