This is the 328th most frequent Russian word.
ночь
"Ночь" means "night" in English.
'Ночь' here is used in its nominative case as the subject of the sentence, referring to the time period between evening and morning.
Ночь была тёплой и спокойной.
The night was warm and calm.
'Ночь' is in its accusative case, indicating duration of an action.
Я провёл всю ночь, читая книгу.
I spent the whole night reading a book.
'Ночью' is used in its instrumental case, meaning 'at night' as a time expression.