This is the 9370th most frequent Turkish word.
yüzünün
"Yüzünün" means "of your face" in English.
The word 'yüzünün' in this sentence means 'of her face' and acts as a possessive construction indicating the face belonging to 'her' (third-person singular).
Onun yüzünün güzelliği beni büyüledi.
The beauty of her face fascinated me.
In this context, 'yüzünün' refers to 'his face' indicating possession and is paired with the concept of expression change to provide specific description.
Yüzünün ifade değişikliği çok ilginçti.
The expression change on his face was very interesting.
Here, 'yüzünün' is used to describe the features or contours of 'your face', showing a second-person singular possessive relationship.