This is the 8529th most frequent Russian word.
ночная
"Ночная" means "night" or "nighttime" (adjective form).
The word 'ночная' here serves as an adjective, describing the gown ('рубашка'), meaning 'night' or 'nocturnal' in context.
Эта ночная рубашка очень удобная.
This nightgown is very comfortable.
In this case, 'ночная' modifies 'прогулка', indicating that the walk occurred during the night.
Ночная прогулка освежила его мысли.
The nighttime walk refreshed his thoughts.
The word 'ночные', the plural form of 'ночная', is used as an adjective qualifying 'дожди', describing rains that fall during the night.