A nightgown is basically a comfy dress you wear for sleeping. Some people prefer pajamas, while others like nightgowns better.
Nightgowns are typically worn by women and girls, although they are closely related to nightshirts, which have long been considered appropriate night wear for people of any gender. Nightgowns often have pretty details like embroidery or lace, and they're made of cotton, silk, or nylon. The word nightgown dates from about 1400.