This is the 143rd most frequent Ukrainian word.
армії
"Армії" in English means "armies."
The word 'армії' is used here in the locative case (на/в кому?) to indicate a location or context within 'the army'.
Вона розповіла про життя в армії.
She shared about life in the army.
The word 'армією' is in the instrumental case (ким? чим?) to show the instrument or means, here 'with the army.'
Я пишаюся армією нашої країни.
I am proud of the army of our country.
The word 'армії' is in the genitive case (чого?) to indicate possession, in this case 'of the army'.