This is the 14093rd most frequent Russian word.
окнах
"Окнах" is the prepositional plural form of "окно," meaning "windows" (used after prepositions).
Here, 'окнах' is used in the locative plural form (prepositional case) indicating the location (on the windows).
На окнах видны красивые узоры от мороза.
Beautiful patterns from frost are visible on the windows.
In this sentence, 'окнах' is not directly present but conceptually refers to windows generally. 'Окна' is used in an instrumental or nominative case.
Через окна можно видеть сад.
Through the windows, one can see the garden.
'Окнах' is used in the locative plural form (prepositional case), describing the location (in the windows).