This is the 11860th most frequent Russian word.
запасного
"Spare" or "reserve."
Here, 'запасное' acts as an adjective describing 'колесо,' emphasizing its role as an additional, redundant item meant for replacement or backup.
В багажнике автомобиля лежит запасное колесо.
In the car trunk lies a spare tire.
In this context, 'запасной' is used to describe 'служба,' referring to it as a reserve or auxiliary service.
Мы встречались с ним во время его запасной службы.
We met him during his reserve service.
Here, 'запасного' modifies 'плана,' indicating that the plan is an alternative or contingency arrangement.