This is the 6599th most frequent Turkish word.
ataşehir
"Ataşehir" is a district in Istanbul, Turkey.
The term 'Ataşehir' here is a proper noun referring to the name of a district in Istanbul, being used as a subject in the sentence.
Ataşehir İstanbul'un modern bir ilçesidir.
Ataşehir is a modern district in Istanbul.
'Ataşehir' is again a proper noun and denotes a specific location (district) within a city, used to specify the place being discussed.
Yeni bir apartman, Ataşehir'deki en yüksek bina olacak.
A new apartment building will be the tallest structure in Ataşehir.
In this sentence, the word 'Ataşehir' acts as the location where the cafes and restaurants are situated.