This is the 16315th most frequent Russian word.
резервные
"Reserve" or "backup."
In this sentence, 'резервные' modifies 'батареи' (batteries), signifying that these batteries are kept as a backup or spare option.
Мы всегда храним резервные батареи для фонариков.
We always keep spare batteries for flashlights.
Here, 'резервные копии' means backup copies, emphasizing copies created to ensure data safety.
Резервные копии данных необходимы для безопасности информации.
Backup copies of data are necessary for information security.
The word 'резервные' refers to 'дни' (days), implying days kept as spare or additional for flexibility.