A person who is unable to drink much alcohol or is sensitive to alcohol (a non-drinker in the sense of being unable to drink).
私は下戸なので、少し飲むだけで顔が赤くなる。
I'm a light drinker, so my face flushes even after a little alcohol.
宴会では下戸の人のためにノンアルコール飲料も用意している。
At parties non-alcoholic drinks are also prepared for people who can't drink.
彼は生まれつきの下戸で、酒に酔いやすい。
He is naturally a light drinker and gets drunk easily.