This is the 14584th most frequent Turkish word.
pencereleri
"Pencereleri" means "their windows" in English.
In this sentence, 'pencerelerini' is used to describe the possession of the windows by the classroom, conjugated to indicate null markers for the plural direct object.
Öğrenciler sınıfın pencerelerini temizledi.
The students cleaned the classroom's windows.
The word 'pencereleri' indicates the plural direct object of the verb 'açtım', referring to windows that are specifically acted upon by the subject.
Evdeki tüm pencereleri açtım.
I opened all the windows in the house.
Here, 'pencereleri' is used to denote the plural direct object of the action performed by the repairman, showing they are the objects worked on.